Import VirtualBox time synchronization driver for NetBSD.

The host time is obtained, but the guest clock is not adjusted yet.
Really adjust the clock.
Do not adjust the clock before the system time is initialized.
Get rid of vtophys(), which is an MD interface.
Ignore a small offset considering time granularity and jitter.
Move from dev/pci to arch/x86/pci.

VirtualBox will not be ported to non-x86 architectures in the
foreseeable future.
Add vboxtime(4) man page.
Update Build and Install section.