Refactoring the sub-leaf handling for CPUID 0x4/0x8000001d to eliminate
a one-off variable and its associated brackets.

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>