copy-before-write: Fix open with child in iothread

copy-before-write: Fix open with child in iothread

The AioContext lock must not be held for bdrv_open_child(), but it is
necessary for the following operations, in particular those using nested
event loops in coroutine wrappers.

Temporarily dropping the main AioContext lock is not necessary because
we know we run in the main thread.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230525124713.401149-9-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>