In cases when the disk frontend was unplugged while a blockjob was
running the blockjob inherits the backing chain. When the blockjob is
then terminated we need to unplug the chain as it will not be used any
more.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>