While meanwhile we at least consume this ourselves, I'm still surprised that we
got away with also not emulating it for HVM guests.

There's now some other (more or less related) cleanup here as well.

1: x86/time: CMOS RTC may run in binary mode
2: time: shorten year determination loop
3: x86/vRTC: the use_timer field is a boolean one
4: x86/vRTC: support century field

Jan