KVM: selftests: Use vcpu_get_cpuid_entry() in CPUID test

Use vcpu_get_cpuid_entry() instead of an open coded equivalent in the
CPUID test.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220614200707.3315957-28-seanjc@google.com