kvm-unit-test: fix cpuid test

If cpuid is called with function value greater than max supported one
max supported value is used instead and index parameter is honored. That
is why we need to initialize index to same value in the test to get
same result from both cpuid invocations.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>