libxl vNUMA PV domain build.

Allows for vNUMA enabled domains to allocate vnodes on physical NUMA
nodes.
Tries to allocate all vnodes on one NUMA node, or on next one if not all
vnodes fit. If no physical numa node found, will let xen decide.

Signed-off-by: Elena Ufimtseva <ufimtseva@gmail.com>