tests: virt: add infrastructure to test getStats

This patch adds the initial infrastructure and the first
very basic tests for the getStats() method.

Change-Id: I28154f54995e2ff9a94c5da6ee38eaa5ed7d9b6d
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26536
Reviewed-by: Dan Kenigsberg <danken@redhat.com>