x86/cpuid: Introduce guest_cpuid() and struct cpuid_leaf

x86/cpuid: Introduce guest_cpuid() and struct cpuid_leaf

Longterm, pv_cpuid() and hvm_cpuid() will be merged into a single
guest_cpuid(), which is also capable of working outside of current context.

To aid this transtion, introduce guest_cpuid() with the intended API, which
simply defers back to pv_cpuid() or hvm_cpuid() as appropriate.

Introduce struct cpuid_leaf which is used to represent the results of a CPUID
query in a more efficient mannor than passing four pointers through the
calltree.

Update all codepaths which should use the new guest_cpuid() API.  These are
the codepaths which have variable inputs, and (other than some specific
x86_emulate() cases) all pertain to servicing a CPUID instruction from a
guest.

The other codepaths using {pv,hvm}_cpuid() with fixed inputs will later be
adjusted to read their data straight from the policy block.

No intended functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Kevin Tian <kevint.tian@intel.com>