virtio-blk: Schedule BH in the right context

The BH must be called in the AioContext of bs. Currently it is only the
main loop, but with coming changes, it could also be a dataplane
IOThread.

Signed-off-by: Fam Zheng <famz@redhat.com>
Tested-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
