qga: Add `guest-get-timezone` command

Adds a new command `guest-get-timezone` reporting the currently
configured timezone on the system. The information on what timezone is
currently is configured is useful in case of Windows VMs where the
offset of the hardware clock is required to have the same offset. This
can be used for management systems like `oVirt` to detect the timezone
difference and warn administrators of the misconfiguration.

Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Sameeh Jubran <sameeh@daynix.com>
Tested-by: Sameeh Jubran <sameeh@daynix.com>
* moved stub implementation to end of function for consistency
* document that timezone names are for informational use only.
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>