KVM: x86: Refactor loop around do_cpuid_func() to separate helper

Move the guts of kvm_dev_ioctl_get_cpuid()'s CPUID func loop to a
separate helper to improve code readability and pave the way for future
cleanup.

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>