virsh: improve doMigrate function docs
virsh: improve doMigrate function docs

When running virsh migrate with --xml option and actual xml file doesn't
exist, virsh hasn't output any error information, although return value
is 1.

* tools/virsh.c: Raising a appropriate error information when operation fails.

* How to reproduce?
% virsh migrate <domain> --live qemu+ssh://<target host>/system --xml non-existent.xml
% echo $?

* Fixed result:
error: file 'non-existent.xml' doesn't exist

Signed-off-by: Alex Jia <e236faec8cb8371bfb245ba3769a84afc9a11438@redhat.com>

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