Explicit use of hugepages with libvirt

Transparent hugepages are nice and easy to get working, but do not survive memory pressure on the host. I/O pressure on the host breaks the 2M pages back to 4k pages, and therefore renders transparent hugepages useless for long-lived processes, as nothing will reassemble the 4k pages back to 2M pages after memory pressure has ceased. Basically, Transparent hugepages are not valid for production environments where memory pressure is normal.

To use hugepages we need to set up 'memorybacking' to 'hugepages' in libvirt.xml. We need also to create a new option flag ('libvirt_hugepages' for example) to toogle the hugepages on and off.

Extra configuration at OS level is needed (AppArmor for example in debian-ite distros).