qemu: blockcopy: Actually unplug unused images when mirror job fails to start

qemu: blockcopy: Actually unplug unused images when mirror job fails to start

If a mirror job fails to start in -blockdev mode we'd not unplug the
backing files we added first because the code on the error path checked
the wrong value. 'rc' is used as status of the code which added the
images, but the state of the 'block(dev)-mirror' call is stored in 'ret'
at that point.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>