block: Consider all child nodes in bdrv_requests_pending()

The function manually recursed into bs->file and bs->backing to check
whether there were any requests pending, but it ignored other children.

There's no need to special case file and backing here, so just replace
these two explicit recursions by a loop recursing for all child nodes.

Reported-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 1446029211-27148-1-git-send-email-kwolf@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
