KVM: selftests: Add dedicated helpers for getting x86 Family and Model

Add dedicated helpers for getting x86's Family and Model, which are the
last holdouts that "need" raw access to CPUID information.  FMS info is
a mess and requires not only splicing together multiple values, but
requires doing so conditional in the Family case.

Provide wrappers to reduce the odds of copy+paste errors, but mostly to
allow for the eventual removal of kvm_get_supported_cpuid_entry().

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221006005125.680782-11-seanjc@google.com