qga: fail early for invalid time

It's possible to set system time with dates after 2070, however, it's
not possible to set the RTC. It has limitation to up to year
2070 (1970+100). In order to keep both clock in sync and before the
kernel complains on invalid values, bail out early.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
