vmd(8): compute i8254 read-back command latch from singular timestamp

vmd(8): compute i8254 read-back command latch from singular timestamp

The intent of the i8254 read-back command is (most likely) to permit
simultaneously latching two or three counters at once along with their
statuses.

To simulate this, we should compute olatch from one timestamp per
read-back command, not one timestamp per counter.

Improved with a tweak by dv@.

Link: https://marc.info/?l=openbsd-tech&m=166213670605453&w=2
ok dv@ mlarkin@