kvm/eventfd: Use priority waitqueue to catch events before userspace

kvm/eventfd: Use priority waitqueue to catch events before userspace

When posted interrupts are available, the IRTE is modified to deliver
interrupts direclty to the vCPU and nothing ever reaches userspace, if
it's listening on the same eventfd that feeds the irqfd.

I like that behaviour. Let's do it all the time, even without posted
interrupts. It makes it much easier to handle IRQ remapping invalidation
without having to constantly add/remove the fd from the userspace poll
set. We can just leave userspace polling on it, and the bypass will...
well... bypass it.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>