The per-VQ MSI-X interrupt handler needs to sync DMA mappings in the

The per-VQ MSI-X interrupt handler needs to sync DMA mappings in the
same way that the shared interrupt handler does.  This is one of the
requirements of virtio_dequeue(), as specified in its comment above.

Without the DMA sync, it will not see a new entry on the ring and
return.  Since the interrupt is edge-triggered there won't be another
one and we'll get stuck.

ok dv@