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>