Instead of individual conditional statements to look for each hypervisor
type, use a table to make it easier to add more in the future, if needed.

Add VirtualBox detection to the table ("VBoxVBoxVBox" is the hypervisor
vendor string to look for.) Also add VM_GUEST_VBOX to the VM_GUEST
enumeration to indicate VirtualBox.

Save the CPUID base for the hypervisor entry that we detected. Driver code
may need to know about it in order to obtain additional CPUID features.

Approved by: bryanv, jhb
Differential Revision: https://reviews.freebsd.org/D16305