mirror: Hold main AioContext lock for calling bdrv_open_backing_file()

bdrv_open_backing_file() calls bdrv_open_inherit(), so all callers must
hold the main AioContext lock.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230525124713.401149-6-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>