[zircon][arch/x86] CPU Features: Minor cleanup

* Add an enumeration entry for when we're not running under a hypervisor;
  today UNKNOWN is used for 'all non-KVM and all physical'.

  This allows code elsewhere in the kernel to see whether we're running
  under a hypervisor or not, so we can apply certain workarounds if
  needed.

* Modify select_microarch_config to take the x86 uarch enum by parameter
  rather than global and have it return the selected config operations
  table.

I'd like to move towards an interface for code throughout the kernel to
query decoded CPUID feature bits/synthetic feature bits/family-model-
stepping data. The interface would allow writing tests for particular
F/M/S targets. Marking the globals as such is a start towards that.

Change-Id: Ic94d7661c8051fbda3ec9eef6e31750e0c53a08d