blockdev: acquire AioContext in blockdev_mark_auto_del()

When an emulated storage controller is unrealized it will call
blockdev_mark_auto_del().  This will cancel any running block job (and
that eventually releases its reference to the BDS so it can be freed).

Since the block job may be executing in another AioContext we must
acquire/release to ensure thread safety.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1413889440-32577-4-git-send-email-stefanha@redhat.com