KVM: selftests: Inline "get max CPUID leaf" helpers

Make the "get max CPUID leaf" helpers static inline, there's no reason to
bury the one liners in processor.c.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220614200707.3315957-38-seanjc@google.com