This lets the RTC get adjustments from the host NTP client.
The watchdog still uses the vm_clock.  The previous behavior is
available with "-rtc clock=vm".

Cc: Andreas Färber <afaerber@suse.de>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>