xen-blkfront: fix resume issues after a migration

xen-blkfront: fix resume issues after a migration

After a migrate to another host (which may not have multiqueue
support), the number of rings (block hardware queues)
may be changed and the ring info structure will also be reallocated.

This patch fixes two related bugs:
 * call blk_mq_update_nr_hw_queues() to make blk-core know the number
   of hardware queues have been changed.
 * Don't store rinfo pointer to hctx->driver_data, because rinfo may be
   reallocated so use hctx->queue_num to get the rinfo structure instead.

Orabug: 23340426
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
(cherry picked from commit 2a6f71ad99cabe436e70c3f5fcf58072cb3bc07f)
Signed-off-by: Bob Liu <bob.liu@oracle.com>