kvm: x86: cpuid: remove the unnecessary variable

The use of local variable *function* is not necessary here. Remove
it to avoid compiling warning with -Wunused-but-set-variable option.

Signed-off-by: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>