virtio_blk: add support for cache flush

Recent qemu has added a VIRTIO_BLK_F_FLUSH flag to advertise that the
virtual disk has a volatile write cache that needs to be flushed.  In case
we see this feature implement tell the Linux block layer about the fact
and use the new VIRTIO_BLK_T_FLUSH to flush the cache when required.  This
allows for an correct and simple implementation of write barriers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>