virtio_pci: fix announcement of MSI-X interrupts for queues

Queues that do not need interrupts - for instance, output queues - do
not have a corresponding entry in vtpci_msix_vq_interrupts.
So, it was wrong to increment a pointer into that array when iterating
over such a queue.

I ran into this bug while trying to use virtio_console(4) that allocates
a lot of queues with every other being an output queue without an
interrupt handler (if MultiplePorts feature is negotiated).

MFC after: 2 weeks