libvirt-guests: Allow time sync on guests resume

libvirt-guests: Allow time sync on guests resume

Well, imagine domains were running, and as the host went down, they
were managesaved. Later, after some time, the host went up again and
domains got restored. But without correct time. And depending on how
long was the host shut off, it may take some time for ntp to sync the
time too. But hey, wait a minute. We have an API just for that! So:

1) Introduce SYNC_TIME variable in libvirt-guests.sysconf to allow
users control over the new functionality
2) Call 'virsh domtime --sync $dom' in the libvirt-guests script.

Unfortunately, this is all-or-nothing approach (just like anything
else with the script). Domains are required to have configured and
running qemu-ga inside.

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