KVM: selftests: x86: Use "this_cpu" prefix for cpu vendor queries

Replace is_intel/amd_cpu helpers with this_cpu_* helpers to better
convey the intent of querying vendor of the current cpu.

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/20230111004445.416840-2-vannapurve@google.com


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