virsh: tweak help output for VSH_OT_DATA
virsh: tweak help output for VSH_OT_DATA

https://bugzilla.redhat.com/show_bug.cgi?id=609044 complained
that 'virsh help pool-create-as' didn't document the shortcut
that you can do 'virsh pool-create-as $name $type --target $target'
rather than having to supply the four optional source- arguments
in order to fill out the necessary positional arguments.

This one-liner changes the help output to hopefully make this more obvious:

  NAME
    pool-create-as - create a pool from a set of args

  SYNOPSIS
    pool-create-as <name> [--print-xml] <type> [<source-host>] [<source-path>] [<source-dev>] [<source-name>] [<target>] [--source-format <string>]

  DESCRIPTION
    Create a pool.

  OPTIONS
    [--name] <string>  name of the pool
    --print-xml      print XML document, but don't define/create
    [--type] <string>  type of the pool
    [--source-host] <string>  source-host for underlying storage
    [--source-path] <string>  source path for underlying storage
    [--source-dev] <string>  source device for underlying storage
    [--source-name] <string>  source name for underlying storage
    [--target] <string>  target for underlying storage
    --source-format <string>  format for underlying storage

* tools/virsh.c (vshCmddefHelp): Make it more obvious that data
arguments may, but not must, be specified by option leaders.

danwent/libvirt-ovs,foomango/libvirt,emaste/libvirt,fabianfreyer/libvirt,emaste/libvirt,eskultety/libvirt,dumbbell/libvirt,bjzhang/libvirt,usc-isi/libvirt,elmarco/libvirt,nertpinx/libvirt,zhlcindy/libvirt-1.1.4-maintain,soulxu/libvirt-xuhj,nertpinx/libvirt,bjzhang/libvirt,kantai/libvirt-vfork,warewolf/libvirt,kantai/libvirt-vfork,zippy2/libvirt,rlaager/libvirt,emaste/libvirt,bjzhang/libvirt,zhlcindy/libvirt-1.1.4-maintain,datto/libvirt,VenkatDatta/libvirt,wiedi/libvirt,rbu/libvirt,eskultety/libvirt,elmarco/libvirt,fabianfreyer/libvirt,foomango/libvirt,danwent/libvirt-ovs,olafhering/libvirt,rbu/libvirt,taget/libvirt,soulxu/libvirt-xuhj,andreabolognani/libvirt,sshah-solarflare/Libvirt-PCI-passthrough-,warewolf/libvirt,agx/libvirt,jeckersb/libvirt,novel/fbsd-libvirt,leilihh/libvirt,nertpinx/libvirt,kantai/libvirt-vfork,leilihh/libvirt,rmarwaha/libvirt,jeckersb/libvirt,fabianfreyer/libvirt,rlaager/libvirt,iam-TJ/libvirt,wiedi/libvirt,wiedi/libvirt,eskultety/libvirt,iam-TJ/libvirt,siboulet/libvirt-openvz,jfehlig/libvirt,elmarco/libvirt,jardasgit/libvirt,warewolf/libvirt,andreabolognani/libvirt,nertpinx/libvirt,foomango/libvirt,taget/libvirt,rbu/libvirt,elmarco/libvirt,warewolf/libvirt,zippy2/libvirt,foomango/libvirt,usc-isi/libvirt,eskultety/libvirt,usc-isi/libvirt,shugaoye/libvirt,leilihh/libvirt,crobinso/libvirt,cbosdo/libvirt,rmarwaha/libvirt1,danwent/libvirt-ovs,andreabolognani/libvirt,leilihh/libvirt,datto/libvirt,leilihh/libvirt,shugaoye/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,wiedi/libvirt,amery/libvirt-vserver,warewolf/libvirt,agx/libvirt,nertpinx/libvirt,amery/libvirt-vserver,jardasgit/libvirt,elmarco/libvirt,olafhering/libvirt,novel/fbsd-libvirt,dumbbell/libvirt,jfehlig/libvirt,warewolf/libvirt,wiedi/libvirt,zhlcindy/libvirt-1.1.4-maintain,novel/fbsd-libvirt,rmarwaha/libvirt,siboulet/libvirt-openvz,sshah-solarflare/Libvirt-PCI-passthrough-,rmarwaha/libvirt1,trainstack/libvirt,VenkatDatta/libvirt,rlaager/libvirt,emaste/libvirt,kantai/libvirt-vfork,eskultety/libvirt,shugaoye/libvirt,jeckersb/libvirt,danwent/libvirt-ovs,dumbbell/libvirt,siboulet/libvirt-openvz,trainstack/libvirt,rmarwaha/libvirt,jardasgit/libvirt,emaste/libvirt,leilihh/libvirt,taget/libvirt,jardasgit/libvirt,cbosdo/libvirt,amery/libvirt-vserver,VenkatDatta/libvirt,danwent/libvirt-ovs,VenkatDatta/libvirt,dumbbell/libvirt,wiedi/libvirt,amery/libvirt-vserver,novel/fbsd-libvirt,jeckersb/libvirt,rmarwaha/libvirt,agx/libvirt,datto/libvirt,rlaager/libvirt,siboulet/libvirt-openvz,jeckersb/libvirt,trainstack/libvirt,novel/fbsd-libvirt,taget/libvirt,agx/libvirt,rbu/libvirt,datto/libvirt,sshah-solarflare/Libvirt-PCI-passthrough-,soulxu/libvirt-xuhj,libvirt/libvirt,trainstack/libvirt,warewolf/libvirt,jeckersb/libvirt,taget/libvirt,zippy2/libvirt,cbosdo/libvirt,bjzhang/libvirt,VenkatDatta/libvirt,emaste/libvirt,trainstack/libvirt,dumbbell/libvirt,cbosdo/libvirt,iam-TJ/libvirt,rmarwaha/libvirt1,rbu/libvirt,soulxu/libvirt-xuhj,emaste/libvirt,novel/fbsd-libvirt,foomango/libvirt,crobinso/libvirt,usc-isi/libvirt,andreabolognani/libvirt,crobinso/libvirt,rmarwaha/libvirt1,olafhering/libvirt,trainstack/libvirt,olafhering/libvirt,usc-isi/libvirt,andreabolognani/libvirt,sshah-solarflare/Libvirt-PCI-passthrough-,rmarwaha/libvirt,novel/fbsd-libvirt,iam-TJ/libvirt,rmarwaha/libvirt1,rmarwaha/libvirt,zhlcindy/libvirt-1.1.4-maintain,siboulet/libvirt-openvz,amery/libvirt-vserver,jfehlig/libvirt,novel/fbsd-libvirt,zhlcindy/libvirt-1.1.4-maintain,shugaoye/libvirt,kantai/libvirt-vfork,bjzhang/libvirt,fabianfreyer/libvirt,libvirt/libvirt,crobinso/libvirt,soulxu/libvirt-xuhj,rlaager/libvirt,agx/libvirt,rmarwaha/libvirt1,iam-TJ/libvirt,novel/fbsd-libvirt,cbosdo/libvirt,dumbbell/libvirt,sshah-solarflare/Libvirt-PCI-passthrough-,wiedi/libvirt,trainstack/libvirt,iam-TJ/libvirt,shugaoye/libvirt,jeckersb/libvirt,libvirt/libvirt,libvirt/libvirt,datto/libvirt,jfehlig/libvirt,zippy2/libvirt,iam-TJ/libvirt