PVH: Set bootmethod to PVH

Now that we can PVH boot on a non-Xen hypervisor, we shouldn't set
machdep.bootmethod to "XEN". Instead, set it to "PVH"; there are
other ways to discern the hypervisor.

Reviewed by: royger
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D36191