MFC r325108:

Improve the performance of the hpet timer in bhyve guests by making the
timer frequency a power of two. This changes the frequency from 10 to
16.7 MHz (2 ^ 24 HZ). Using a power of two avoids roundoff errors when
doing arithmetic in sbintime_t units.

Testing shows this can fix erratic ntpd behavior in guests using the
hpet timer (which is the default for multicore guests).