KVM: Allow KVM_CHECK_EXTENSION on the vm fd

The KVM_CHECK_EXTENSION is only available on the kvm fd today. Unfortunately
on PPC some of the capabilities change depending on the way a VM was created.

So instead we need a way to expose capabilities as VM ioctl, so that we can
see which VM type we're using (HV or PR). To enable this, add the
KVM_CHECK_EXTENSION ioctl to our vm ioctl portfolio.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>