hpet: remove muldiv64()

hpet: remove muldiv64()

hpet defines a clock period in femtoseconds but
then converts it to nanoseconds to use the internal
timers.

We can define the period in nanoseconds and use it
directly, this allows to remove muldiv64().

We only need to convert the period to femtoseconds
to put it in internal hpet capability register.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>