virsh: Add VSH_OFLAG_EMPTY_OK for attach-disk command
virsh: Add VSH_OFLAG_EMPTY_OK for attach-disk command

As the description of removing CDROM media from
  http://wiki.libvirt.org/page/QEMUSwitchToLibvirt#eject_DEV

Add flag 'VSH_OFLAG_EMPTY_OK' to the option 'source' of attach-disk

Then avoid outputting <source> in the XML if 'source' was empty,
rather than trusting libvirt domain_conf.c to understand an empty
string.

Signed-off-by: Xu He Jie <7d5f3c8f6062b24a532cb48ceb5119203405f485@linux.vnet.ibm.com>
Signed-off-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

eskultety/libvirt,siboulet/libvirt-openvz,trainstack/libvirt,trainstack/libvirt,rlaager/libvirt,trainstack/libvirt,usc-isi/libvirt,zippy2/libvirt,warewolf/libvirt,eskultety/libvirt,warewolf/libvirt,olafhering/libvirt,rmarwaha/libvirt1,eskultety/libvirt,libvirt/libvirt,rmarwaha/libvirt1,rmarwaha/libvirt,rmarwaha/libvirt,novel/fbsd-libvirt,elmarco/libvirt,fabianfreyer/libvirt,crobinso/libvirt,danwent/libvirt-ovs,crobinso/libvirt,jardasgit/libvirt,warewolf/libvirt,rmarwaha/libvirt,rmarwaha/libvirt1,nertpinx/libvirt,zhlcindy/libvirt-1.1.4-maintain,leilihh/libvirt,wiedi/libvirt,trainstack/libvirt,cbosdo/libvirt,siboulet/libvirt-openvz,jfehlig/libvirt,danwent/libvirt-ovs,shugaoye/libvirt,datto/libvirt,trainstack/libvirt,taget/libvirt,usc-isi/libvirt,jardasgit/libvirt,leilihh/libvirt,zhlcindy/libvirt-1.1.4-maintain,novel/fbsd-libvirt,warewolf/libvirt,jeckersb/libvirt,emaste/libvirt,jfehlig/libvirt,rlaager/libvirt,elmarco/libvirt,nertpinx/libvirt,andreabolognani/libvirt,datto/libvirt,eskultety/libvirt,rmarwaha/libvirt1,cbosdo/libvirt,andreabolognani/libvirt,wiedi/libvirt,bjzhang/libvirt,iam-TJ/libvirt,jeckersb/libvirt,dumbbell/libvirt,danwent/libvirt-ovs,datto/libvirt,libvirt/libvirt,siboulet/libvirt-openvz,emaste/libvirt,cbosdo/libvirt,cbosdo/libvirt,taget/libvirt,novel/fbsd-libvirt,olafhering/libvirt,usc-isi/libvirt,VenkatDatta/libvirt,trainstack/libvirt,siboulet/libvirt-openvz,wiedi/libvirt,rlaager/libvirt,taget/libvirt,foomango/libvirt,danwent/libvirt-ovs,emaste/libvirt,fabianfreyer/libvirt,dumbbell/libvirt,jeckersb/libvirt,agx/libvirt,nertpinx/libvirt,wiedi/libvirt,VenkatDatta/libvirt,warewolf/libvirt,emaste/libvirt,andreabolognani/libvirt,agx/libvirt,bjzhang/libvirt,jardasgit/libvirt,wiedi/libvirt,jeckersb/libvirt,dumbbell/libvirt,rlaager/libvirt,rmarwaha/libvirt1,datto/libvirt,rmarwaha/libvirt,bjzhang/libvirt,agx/libvirt,jardasgit/libvirt,elmarco/libvirt,novel/fbsd-libvirt,shugaoye/libvirt,iam-TJ/libvirt,andreabolognani/libvirt,cbosdo/libvirt,siboulet/libvirt-openvz,iam-TJ/libvirt,bjzhang/libvirt,usc-isi/libvirt,iam-TJ/libvirt,zippy2/libvirt,rmarwaha/libvirt,fabianfreyer/libvirt,zhlcindy/libvirt-1.1.4-maintain,fabianfreyer/libvirt,zhlcindy/libvirt-1.1.4-maintain,novel/fbsd-libvirt,foomango/libvirt,olafhering/libvirt,crobinso/libvirt,zippy2/libvirt,iam-TJ/libvirt,nertpinx/libvirt,agx/libvirt,nertpinx/libvirt,jfehlig/libvirt,novel/fbsd-libvirt,bjzhang/libvirt,iam-TJ/libvirt,trainstack/libvirt,VenkatDatta/libvirt,shugaoye/libvirt,shugaoye/libvirt,warewolf/libvirt,olafhering/libvirt,fabianfreyer/libvirt,agx/libvirt,dumbbell/libvirt,jfehlig/libvirt,libvirt/libvirt,shugaoye/libvirt,rmarwaha/libvirt,dumbbell/libvirt,leilihh/libvirt,wiedi/libvirt,jeckersb/libvirt,elmarco/libvirt,novel/fbsd-libvirt,libvirt/libvirt,emaste/libvirt,emaste/libvirt,VenkatDatta/libvirt,jeckersb/libvirt,foomango/libvirt,iam-TJ/libvirt,leilihh/libvirt,zippy2/libvirt,taget/libvirt,wiedi/libvirt,rlaager/libvirt,andreabolognani/libvirt,novel/fbsd-libvirt,taget/libvirt,leilihh/libvirt,warewolf/libvirt,crobinso/libvirt,eskultety/libvirt,VenkatDatta/libvirt,rmarwaha/libvirt1,datto/libvirt,emaste/libvirt,foomango/libvirt,foomango/libvirt,leilihh/libvirt,zhlcindy/libvirt-1.1.4-maintain,novel/fbsd-libvirt,jardasgit/libvirt,elmarco/libvirt,usc-isi/libvirt,dumbbell/libvirt,jeckersb/libvirt,danwent/libvirt-ovs