block: Move guest_block_size into BlockBackend guest_block_size is a guest device property so it should be moved into the interface between block layer and guest devices, which is the BlockBackend. Signed-off-by: Max Reitz <mreitz@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Reviewed-by: Alberto Garcia <berto@igalia.com> Reviewed-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>