libvirt-guests: Initialize SYNC_TIME
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1191227
    
    Since 0fa15b19 we have this variable SYNC_TIME which allows users to
    synchronize time on domain resume. However, despite what documentation
    says, it's by default on because it's never initialized. Fix this by
    setting it to zero at the beginning of the libvirt-guests script.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>