All block drivers that can have child nodes implement .bdrv_child_perm()
now. Make this officially a requirement by asserting that only drivers
without children can omit .bdrv_child_perm().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Acked-by: Fam Zheng <famz@redhat.com>