Rework hypervisor detection

[John Baldwin]

The current code to detect a hypervisor in FreeBSD is a git gross.  First, 
it's x86-specific, but stored in MI code.  Second, it does not follow the best 
practice of using cpuid info but only depends on DMI/SMBIOS strings.  The 
patch below attempts to improve this.  We might want to move the VM_GUEST_HV 
detection into this new function as well.

https://reviews.freebsd.org/D1010

-- 
John Baldwin