libxl: implement virDomainGetNumaParameters

libxl: implement virDomainGetNumaParameters

Although, having it depending on Xen >= 4.3 (by using the proper
libxl feature flag).

Xen currently implements a NUMA placement policy which is basically
the same as the 'interleaved' policy of `numactl', although it can
be applied on a subset of the available nodes. We therefore hardcode
"interleave" as 'numa_mode', and we use the newly introduced libxl
interface to figure out what nodes a domain spans ('numa_nodeset').

With this change, it is now possible to query the NUMA node
affinity of a running domain:

[raistlin@Zhaman ~]$ sudo virsh --connect xen:/// list
 Id    Name                           State
----------------------------------------------------
 23    F18_x64                        running

[raistlin@Zhaman ~]$ sudo virsh --connect xen:/// numatune 23
numa_mode      : interleave
numa_nodeset   : 1

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>