virtio_blk: blk-mq support

Switch virtio-blk from the dual support for old-style requests and bios
to use the block-multiqueue.

Acked-by: Asias He <asias@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christoph Hellwig <hch@lst.de>