The older detection methods (smbios.bios.vendor and smbios.system.product)
are able to determine some virtual machines, but the vm_guest variable was
still only being set to VM_GUEST_VM.

Since we do know what some of them specifically are, we can set vm_guest
appropriately.

Also, if we see the CPUID has the HV flag, but we were unable to find a
definitive vendor in the Hypervisor CPUID Information Leaf, fall back to
the older detection methods, as they may be able to determine a specific
HV type.

Add VM_GUEST_PARALLELS value to VM_GUEST for Parallels.

Approved by: cem
Differential Revision: https://reviews.freebsd.org/D20305