I'm surprised that we didn't consume this thus far, and that we got away with
also not emulating it for HVM guests.

The 1st and 3rd patches are imo candidates for 4.22, whereas the others (more
or less associated cleanup) likely aren't.

1: x86/time: use RTC century byte when available
2: x86/time: move BCD_TO_BIN() uses
3: x86/vRTC: support century field
4: x86/vRTC: use available macros for BCD <-> BIN conversion
5: tools/xen-hvmctx: shorten various format strings a little

Jan