vhost/blk: set blkcfg->num_queues to the max available queues number

QEMU overrides this value anyway, but other
virtio implementation may not. Setting
blkcfg->num_queues to its target value
won't break anything, and will help out
SPDK virtio-blk driver implementation.

For equivalent field in virtio-scsi, the
Virtio 1.0 spec says:

```
num_queues
is the total number of request virtqueues exposed by the device. The
driver MAY use only one request queue, or it can use more to achieve
better performance.
```

Multiqueue for virtio-blk is oficially still
unsupported, so the blkcfg->num_queues remains
undocumented. Let's implement it adequately
to the virtio-scsi spec.

Change-Id: Ief520385997bb7e745ed17501a972c55955346d2
Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/399121


Reviewed-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>