Rework virtual machine hypervisor detection.

Rework virtual machine hypervisor detection.
- Move the existing code to x86/x86/identcpu.c since it is x86-specific.
- If the CPUID2_HV flag is set, assume a hypervisor is present and query
  the 0x40000000 leaf to determine the hypervisor vendor ID.  Export the
  vendor ID and the highest supported hypervisor CPUID leaf via
  hv_vendor[] and hv_high variables, respectively.  The hv_vendor[]
  array is also exported via the hw.hv_vendor sysctl.
- Merge the VMWare detection code from tsc.c into the new probe in
  identcpu.c.  Add a VM_GUEST_VMWARE to identify vmware and use that in
  the TSC code to identify VMWare.

Differential Revision:	https://reviews.freebsd.org/D1010
Reviewed by:	delphij, jkim, neel