block: create bdrv_get_backing_file_depth() Create bdrv_get_backing_file_depth() in order to be able to show in QMP and HMP how many ancestors backing an image a block device have. Signed-off-by: Benoit Canet <benoit@irqsave.net> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>