numatune: Support NUMA memory tuning in qemu driver

numatune: Support NUMA memory tuning in qemu driver

Implemented as setting NUMA policy between fork and exec as a hook,
using libnuma. Only support memory tuning on domain process currently.

For the nodemask out of range, will report soft warning instead of
hard error in libvirt layer. (Kernel will be silent as long as one
of set bit in the nodemask is valid on the host. E.g. For a host
has two NUMA nodes, kernel will be silent for nodemask "01010101").
So, soft warning is the only thing libvirt can do, as one might want
to specify the numa policy prior to a node that doesn't exist yet,
however, it may come as hotplug soon.