virt: sampling: factor numa stats in a method

This patch moves the NUMA stats collection into
an helper method, to make the code more consistent
and a bit tidier.
No functional changes.

Change-Id: Ifd78ed5637e21cf321c88ddcfe0801fd8d1e90d5
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33782
Reviewed-by: Dan Kenigsberg <danken@redhat.com>