kvm_clock: KVM paravirtual clock support

Add support for the KVM paravirtual clock device.

Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D29733

(cherry picked from commit 6c69c6bb4c7ffde48b130df707799d4eca21ca9f)