Improve the precision of bhyve's vPIT.

Use 'struct bintime' instead of 'sbintime_t' to manage times in vPIT
to postpone rounding to final results rather than intermediate
results. In tests performed by Joyent, this reduced the error measured
by Linux guests by 59 ppm.

Smart OS bug: https://smartos.org/bugview/OS-6923
Submitted by: Patrick Mooney
Reviewed by: rgrimes
Obtained from: SmartOS / Joyent
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20335