xen: vnuma support for PV guests running as domU

Issues Xen hypercall subop XENMEM_get_vnumainfo and sets the
NUMA topology, otherwise sets dummy NUMA node and prevents
numa_init from calling other numa initializators as they dont
work with pv guests.

Signed-off-by: Elena Ufimtseva <ufimtseva@gmail.com>
xen: enable vnuma for PV guest

Enables numa if vnuma topology hypercall is supported and it is domU.

Signed-off-by: Elena Ufimtseva <ufimtseva@gmail.com>
Merge branch 'master' of https://github.com/cosmoecho/linux