mc146818rtc: register the clock reset notifier on the right clock

Commit 884f17c2 (aio / timers: Convert rtc_clock to be a QEMUClockType,
2013-08-21) erroneously changed an occurrence of rtc_clock to
QEMU_CLOCK_REALTIME, which broke the RTC reset notifier in
mc146818rtc.  Fix this.

I redid the patch myself since the original reporter did not sign
off on his.

Cc: qemu-stable@nongnu.org
Reported-by: Lb peace <peaceustc@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>