Export the cpu nodes and sockets from Xen

This is a hand-picked forward patch of commit 1755 on the 1.2 branch
(hand-picked since the trees diverged too much since then):

    The patch changed the xen hypervisor to compute the number of cpu
    sockets/nodes and enables the command line and the RAPI to show this
    information (for RAPI is enabled by default in node details, for gnt-one
    one can use the new “cnodes” and “csockets” fields).

    Originally-Reviewed-by: ultrotter

For the KVM and fake hypervisors, the patch just exports 1 for both
nodes and sockets. This can be fixed, by looking at the
/sys/devices/system/cpu/cpuN/topology directories, and computing the
actual information, but that should be done in a separate patch.

Reviewed-by: imsnah