Hypervisor check fix

Due to various options that can be defined by the user when
creating a vm, the hypervisor type is properly retrieved using
the CPUID instruction.

Hyper-V specs: http://www.microsoft.com/en-us/download/confirmation.aspx?id=39289
VMWare specs: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1009458