scsi: virtio_scsi: Add mq_poll support

This adds polling support to virtio-scsi. It's based on and works similar
to virtblk support where we add a module param to specify the number of
poll queues then subtract to calculate the IO queues.

When using 8 poll queues and a vhost worker per queue we see 4K IOPs
with fio:

fio --filename=/dev/sda --direct=1 --rw=randread --bs=4k \
--ioengine=io_uring --hipri --iodepth=128  --numjobs=$NUM_JOBS

increase like:

jobs	base	poll
1	207K	296K
2	392K	552K
3	581K	860K
4	765K	1235K
5	936K	1598K
6	1104K	1880K
7	1253K	2095K
8	1311k	2187K

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Message-Id: <20231214052649.57743-1-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>