x86/time: reduce rounding errors in calculations

x86/time: reduce rounding errors in calculations

Plain (unsigned) integer division simply truncates the results. The
overall errors are smaller though if we use proper rounding. (Extend
this to the purely cosmetic aspect of time.c's freq_string(), which
before this change I've frequently observed to report e.g. NN.999MHz
HPET clock speeds.)

While adding the rounding logic, also switch to using an unsigned
constant for the other, original half of bus_cycle's calculation.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>