block-backend: Fix blk_new_open() for iothreads

This fixes blk_new_open() to not assume that bs is in the main context.

In particular, the BlockBackend must be created with the right
AioContext because it will refuse to move to a different context
afterwards. (blk->allow_aio_context_change is false.)

Use this opportunity to use blk_insert_bs() instead of duplicating the
bdrv_root_attach_child() call. This is consistent with what
blk_new_with_bs() does. Add comments to document the locking rules.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230525124713.401149-5-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>