block: simplify bdrv_find_base() and bdrv_find_overlay() This simplifies the function bdrv_find_overlay(). With this change, bdrv_find_base() is just a subset of usage of bdrv_find_overlay(), so this also takes advantage of that. Reviewed-by: Eric Blake <eblake@redhat.com> Reviewed-by: Benoit Canet <benoit@irqsave.net> Signed-off-by: Jeff Cody <jcody@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>