automatic IRQ affinity for virtio V3

[Christoph Hellwig]

Hi Michael, hi Jason,

This patches applies a few cleanups to the virtio PCI interrupt handling
code, and then converts the virtio PCI code to use the automatic MSI-X
vectors spreading, as well as using the information in virtio-blk
and virtio-scsi to automatically align the blk-mq queues to the MSI-X
vectors.

Changes since V2:
 - remove a redundant callback check
 - calculate ->msix_vectors correctly
 - add a few Reviewed-by: tags for Jason

Changes since V1:
 - dropped the patches already merged for 4.10-rc
 - new patch to remove struct virtio_pci_vq_info
 - improve probe error handling by unwinding step by step
 - new patch to convert virtio-scsi