qemu: blockjob: Print correct file name in error message

qemu: blockjob: Print correct file name in error message

When attempting a blockcommit from the top layer, the base argument
passed is NULL. This will be dereferenced when attempting a commit with
an empty image chain. Output the real volume path instead:

virsh blockcommit --verbose --path vda --domain DOMNAME --wait
error: invalid argument: top '/path/somefile' in chain for 'vda' has no backing file

instead of:

error: invalid argument: top '(null)' in chain for 'vda' has no backing file