virtio-scsi: Add virtqueue_size parameter allowing virtqueue size to be set.

virtio-scsi: Add virtqueue_size parameter allowing virtqueue size to be set.

Since Linux switched to blk-mq as the default in Linux commit
5c279bd9e406 ("scsi: default to scsi-mq"), virtio-scsi LUNs consume
about 10x as much guest kernel memory.

This commit allows you to choose the virtqueue size for each
virtio-scsi-pci controller like this:

  -device virtio-scsi-pci,id=scsi,virtqueue_size=16

The default is still 128 as before.  Using smaller virtqueue_size
allows many more disks to be added to small memory virtual machines.
For a 1 vCPU, 500 MB, no swap VM I observed:

  With scsi-mq enabled (upstream kernel):              175 disks
    -"- ditto -"-   virtqueue_size=64:                 318 disks
    -"- ditto -"-   virtqueue_size=16:                 775 disks
  With scsi-mq disabled (kernel before 5c279bd9e406): 1755 disks

Note that to have any effect, this requires a kernel patch:

  https://lkml.org/lkml/2017/8/10/689

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Message-Id: <20170810165255.20865-1-rjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>