vfio-pci: Add KVM INTx acceleration

This makes use of the new level irqfd support enabling bypass of qemu
userspace both on INTx injection and unmask.  This significantly
boosts the performance of devices making use of legacy interrupts (ex.
~60% better netperf TCP_RR scores for an e1000e assigned to a Linux
guest and booted with pci=nomsi).  This also avoids flipping mmaps on
and off to simulate EOIs, so greatly improves performance of device
access in addition to interrupt latency.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
