x86/cpu: introduce an optional hook for early hypervisor initialization

Hypervisor detection is done very early on x86, and so can be used to also do
some very early hypervisor related initialization. Such initialization is
required when running as a Xen PVH guest, as for example the PIT needs to be
replaced with an hypervisor based timecounter.

Introduce an optional hook that gets called as part of the early hypervisor
detection.

No functional change intended.

Sponsored by: Cloud Software Group
Reviewed by: markj kib
Differential revision: https://reviews.freebsd.org/D43763