acpi_hpet: Make use of enum for vm_guest to improve readability

No functional change intended.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D44402

(cherry picked from commit 579cb41b132f532bf4915121c0d0b2f43688242e)
(cherry picked from commit ddd779a062c1de28a3f632ece9fe4d1abf07dbdb)