Introduce QEMU_CLOCK_HOST

Despite its name QEMU_CLOCK_REALTIME is (normally) not using
CLOCK_REALTIME / the host system time as base. In order to allow also
non-trivial RTC emulations (MC146818) to follow the host time instead of
the virtual guest time, introduce the new clock type QEMU_CLOCK_HOST. It
is unconditionally based on CLOCK_REALTIME, thus will follow system time
changes of the host.

The only limitation of its current implementation is that pending
host_clock timers may not fire early if the host time is pushed forward
beyond their expiry. So far no urgent need to overcome this limitation
was identified, so it's left as simple as it is (expiry on next alarm
timer tick).

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