KVM: x86: Drop explicit @func param from ->set_supported_cpuid()

Drop the explicit @func param from ->set_supported_cpuid() and instead
pull the CPUID function from the relevant entry.  This sets the stage
for hardening guest CPUID updates in future patches, e.g. allows adding
run-time assertions that the CPUID feature being changed is actually
a bit in the referenced CPUID entry.

No functional change intended.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>