sysutils/kvmclock-kmod: new port - call for testing

Improved performance on KVM paravirtualised systems. Testing
welcomed; please report successes and issues to:

https://reviews.freebsd.org/D29733

kvmclock-kmod is experimental and currently under development. This port
provides an easy and quick method for users to test this code for early
testing, feedback and bug reports.

This driver enables FreeBSD to use a more efficient paravirtualized
hardware clock, instead of emulating one, or abusing hypervisor
interrupts, when running as a virtualized OS under Linux KVM
(Kernel-based Virtual Machine).

Reviewed by: allanjude
Sponsored by: Klara Inc.
Sponsored by: SkunkWerks, GmbH
Differential Revision: https://reviews.freebsd.org/D30459
PR: 216759