virtio_pci irq handling cleanups

[Christoph Hellwig]

Hi Michael,

this series contains a couple cleanups for the virtio_pci interrupt
handling code, including a switch to the new pci_irq_alloc_vectors
helper.  All these are in preparation of taking advantage of the new
PCI layer / core IRQ interrupt affinity handling, for which I will
send out a series once this and some core interrupt handling changes
are in.