Fixes RTC bug with base datetime shifts in clock=vm

Fixes RTC bug with base datetime shifts in clock=vm

This makes all current "-rtc" option parameters combinations produce
fixed/unambiguous RTC timedate reference for hardware emulation
frontends.
It restores determinism of guest execution when used with clock=vm and
specified base <datetime> value.

Buglink: https://bugs.launchpad.net/qemu/+bug/1797033
Signed-off-by: Artem Pisarenko <artem.k.pisarenko@gmail.com>
Message-Id: <1d963c3e013dfedafa1f6edb9fb219b7e49e39da.1539846575.git.artem.k.pisarenko@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>