blk_insert_bs() requires that the caller holds the AioContext lock for
the node to be inserted. Since commit c066e808, neglecting to do so
causes a crash when the child has to be moved to a different AioContext
to attach it to the BlockBackend.

This fixes qmp_blockdev_insert_anon_medium(), which is called for the
QMP commands 'blockdev-insert-medium' and 'blockdev-change-medium', to
correctly take the lock.

Cc: qemu-stable@nongnu.org
Fixes: https://issues.redhat.com/browse/RHEL-3922


Fixes: c066e808
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20231013153302.39234-2-kwolf@redhat.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>