timer_management: Get correct UTC and timezone time

    1. Both 'date -u +%s' and 'time.time()' will get localtime(timezone time)
       instead of UTC time.
    2. 'time.mktime()' output is platform-dependent, and affected by daylight
       and timezone, which means translate timestr on different boxes may have
       different results. Here fetch both UTC and timezone time in the same
       format and translate on the same box to get relative gap.
    3. For 'offset=timezone' setting, the host timezone should be set before
       the guest active.
Signed-off-by: Yanbing Du <ydu@redhat.com>
I'm not clear why the two vector time should be added.
Take an example:
```
span1 = vector(-2) * 3600
span2 = vector(-4) * 3600
```
Shouldn't the gap be `span1 sub span2` or likewise?