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