PPC: KVM: Use vm check_extension for pv hcall

To find out whether we support the KVM hypercall interface we need to ask KVM
on the VM level rather than the global KVM level, because Book3S HV KVM does
not support it and we play conservative when both HV and PR are loaded.

So instead, use the VM helper that falls back to global KVM enumeration. That
should cover all cases.

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