PVH: Set bootmethod to XEN or PVH appropriately

[cperciva]

Summary

Now that we can PVH boot on a non-Xen hypervisor, we shouldn't set
machdep.bootmethod to "XEN" unconditionally. Instead, set it to
"XEN" if we're running under Xen and "PVH" otherwise.

[imp]

I'm assuming that the whole issue of overloading boot method with what bios interface is available isn't made worse by this :)

[cperciva]

In D36191#821464, @imp wrote:

I'm assuming that the whole issue of overloading boot method with what bios interface is available isn't made worse by this :)

Right, in this case we're not really booting with PC BIOS *or* UEFI.

I don't think it really matters here, but it caught my eye when I ran sysctl -a | grep -i xen so I figured I might as well fix it. In the longer term I expect this file to move to sys/x86/x86/pvh.c so we can more fully separate "PVH" and "Xen PVH", so this code will be churned later anyway.

[royger]

I think I would prefer setting it to PVH in both cases, we have other ways to expose the underlying hypervisor, and the boot entry point is the same for Xen or KVM PVH.

[cperciva]

Set bootmethod to "PVH" even when we're PVH-booting from Xen, per royger's comment.

[royger]

LGTM, the commit/revision title needs updating AFAICT.