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:	2fee875629 ("abstract out the vm detection via smbios..")
Differential Revision:	https://reviews.freebsd.org/D39140

sys/dev/smbios/smbios_subr.c

@@ -69,6 +69,14 @@ identify_hypervisor_smbios(void)
 	char *p;
 	int i;
 
+	/*
+	 * Some platforms, e.g., amd64, have other ways of detecting what kind
+	 * of hypervisor we may be running under.  Make sure we don't clobber a
+	 * more specific vm_guest that's been previously detected.
+	 */
+	if (vm_guest != VM_GUEST_NO && vm_guest != VM_GUEST_VM)
+		return;
+
 	/*
 	 * XXX: Some of these entries may not be needed since they were
 	 * added to FreeBSD before the checks above.