Investigate using IOEVENTFD to avoid VM exits

[n0toose]

This could help us do some tasks asynchronously and improve performance; the "how" is pretty unclear to me at the moment, however.

See:

• https://wiki.qemu.org/Features/VirtioIoeventfd
• https://docs.kernel.org/virt/kvm/api.html#kvm-ioeventfd