KVM: Add helper to run KVM_CHECK_EXTENSION on vm fd

We now can call KVM_CHECK_EXTENSION on the kvm fd or on the vm fd, whereas
the vm version is more accurate when it comes to PPC KVM.

Add a helper to make the vm version available that falls back to the non-vm
variant if the vm one is not available yet to stay compatible.

Signed-off-by: Alexander Graf <agraf@suse.de>