Enable host-clock-based RTC

Enable host-clock-based RTC

Switch RTC emulations to the new host_clock instead of vm_clock by
default. This has the advantage that the emulated RTC will follow
automatically the host time while it might be tuned via NTP. vm_clock
can still be selected by passing '-rtc clock=vm' on the command line.

Note that some RTC emulations (at least M48T59) already use the host
time unconditionally while others (namely MC146818) do not. This patch
introduces the required infrastructure for selecting the base clock but
only converts MC146818 for now.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>