x86: gate smbios hypervisor identification behind vm_guest

cpuid detection may have picked up a more specific guest type already,
and a follow-up check of smbios vendor/product may erroneously blow
away the previously detected type.

This reportedly fixes the boot under Hyper-V, which advertises an
smbios.system.product of "Virtual Machine."

PR: 270239
Reviewed by: imp, kib (both earlier version, same concept)
Fixes: 2fee87562948 ("abstract out the vm detection via smbios..")
Differential Revision: https://reviews.freebsd.org/D39140