Description of problem:
SMP guests experience increased I/O latencies when the application submits a request on one vcpu and the completion interrupt is handled by another vcpu.  When this happens an Inter-Processor Interrupt (IPI) is required so the second vcpu can wake up the first vcpu.

Users can pin the application to specific vcpus to avoid IPIs, but a cleaner solution that does not involve manual configuration is virtio-blk/scsi multi-queue.

By providing additional virtqueues (one per vcpu) we avoid the need handle completion interrupts on a different vcpu.  Therefore IPIs are not needed and request latency is improved.

QEMU should set the -device virtio-blk,num-queues=N and -device virtio-scsi,num_queues=N properties to the number of vcpus by default.  This will result in additional MSI-X vectors being allocated but otherwise has no drawbacks.

How reproducible:
100%

Steps to Reproduce:
1. Launch a guest with 4 vcpus, latest machine type, one virtio-blk device, and one virtio-scsi device.
2. guest# ls /sys/block/vda/mq | wc -l
3. guest# ls /sys/block/sda/mq | wc -l

Actual results:
2. guest# ls /sys/block/vda/mq | wc -l
1
3. guest# ls /sys/block/sda/mq | wc -l
1

Expected results:
2. guest# ls /sys/block/vda/mq | wc -l
4
3. guest# ls /sys/block/sda/mq | wc -l
4