The aim of this series is to allow using the emulated PC RTC (MC146818)
as a reliable time source for guests. This is particularly useful if the
host runs NTP or has otherwise access to an accurate clock while the
guest has not (no network, impossible to add an NTP implementation
etc.).
To achieve this, the command line switch -rtc is introduced. It takes
the option 'clock' to switch between the currently used base ('vm') and
the new QEMU_CLOCK_HOST ('host'). At this chance, -rtc is also used to
deprecate all the other RTC-related stand-alone switches.
First tests indicate that this approach works as expected and could
increase the usefulness of the virtual RTC enormously. However, there
might be pitfalls I've missed so far. Feedback would be welcome!