xen-blkfront: convert to blk-mq APIs

Note: This patch is based on original work of Arianna's internship for
GNOME's Outreach Program for Women.

Only one hardware queue is used now, so there is no significant
performance change

The legacy non-mq code is deleted completely which is the same as other
drivers like virtio, mtip, and nvme.

Also dropped one unnecessary holding of info->io_lock when calling
blk_mq_stop_hw_queues().

Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jens Axboe <axboe@fb.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>