Whether the custom RTC setting is enabled shouldn't in itself
affect determinism (as long as the actual RTC value is properly
synced). Alters the logic added in 4b2906c.
I'm not entirely certain that this is correct, but the current
code doesn't really make sense to me... If we need to force the
RTC bias to 0 when custom RTC is enabled, why don't we need to
do it when custom RTC is disabled? The code for getting the
host system's current time doesn't contain any special handling
for the guest's RTC bias as far as I can tell.
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