vm iface statistics: never report negative rates

When Linux tx/rx couters overflow their 32 bit limit, they wrap back to
zero. This causes our reported rxRate/txRate to become negative for one
nample. 51073875161d9b932b7

Change-Id: I40daeb12172a7f54f135b3018405ef228983fa93
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23598
Tested-by: Ondřej Svoboda <osvoboda@redhat.com>