KVM VMX and x86 PAT MSR macro cleanup for 6.12:

 - Add common defines for the x86 architectural memory types, i.e. the types
   that are shared across PAT, MTRRs, VMCSes, and EPTPs.

 - Clean up the various VMX MSR macros to make the code self-documenting
   (inasmuch as possible), and to make it less painful to add new macros.