vfio/quirks: Enable ioeventfd quirks to be handled by vfio directly

vfio/quirks: Enable ioeventfd quirks to be handled by vfio directly

With vfio ioeventfd support, we can program vfio-pci to perform a
specified BAR write when an eventfd is triggered.  This allows the
KVM ioeventfd to be wired directly to vfio-pci, entirely avoiding
userspace handling for these events.  On the same micro-benchmark
where the ioeventfd got us to almost 90% of performance versus
disabling the GeForce quirks, this gets us to within 95%.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>