KVM: selftests: x86: Cache host CPU vendor (AMD vs. Intel)

Cache the host CPU vendor for userspace and share it with guest code.

All the current callers of this_cpu* actually care about host cpu so
they are updated to check host_cpu_is*.

Suggested-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Vishal Annapurve <vannapurve@google.com>
Link: https://lore.kernel.org/r/20230111004445.416840-3-vannapurve@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>