libx86: Introduce lib/x86/cpuid.h

libx86: Introduce lib/x86/cpuid.h

Begin to untangle the header dependency tangle by moving definition of
struct cpuid_leaf out of x86_emulate.h into the new cpuid.h.

Additionally, plumb the header through to libxc.  This is technically a
redundant include at this point, but it helps build-test the later changes,
and will be used eventually.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>