Add a workaround to the hypervisor detection for older versions of KVM.

Originally KVM set %eax to 0 in the cpuid leaf 0x4000000 rather than
to the highest supported leaf in the hypervisor "branch". Detect this
case and fixup the %eax value so that the hypervisor is still
detected.

Reported by: jpaetzel
Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D14810