Before:

 # virsh blockjob r7 vdc
 error: An error occurred, but the cause is unknown

After:

 # virsh blockjob r7 vdc
 error: Disk 'vdc' not found in the domain

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1241355Signed-off-by: NLuyao Huang <lhuang@redhat.com>