virNetDevOpenvswitchInterfaceStats: Be more forgiving when fetching stats

virNetDevOpenvswitchInterfaceStats: Be more forgiving when fetching stats

https://bugzilla.redhat.com/show_bug.cgi?id=1461270

When fetching stats for a vhost-user type of interface, we run
couple of ovs-vsctl commands and parse their output. However, not
all stats exist at all times, for instance "rx_dropped" or
"tx_errors" can be missing. Thing is, we ask for a bulk of
statistics and if one of them is missing an error is reported
instead of returning the rest. Since we ignore errors, we fail to
set statistics. Fix this by asking for each piece alone.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>