KVM: PPC: Override host vmx/vsx/dfp only when information known

The -cpu host feature tries to find out the host capabilities based
on device tree information. However, we don't always have that available
because it's an optional property in dt.

So instead of force unsetting values depending on an unreliable source
of information, let's just try to be clever about it and not override
capabilities when we don't know the device tree pieces.

This fixes altivec with -cpu host on YDL PowerStations.

Reported-by: Nishanth Aravamudan <nacc@us.ibm.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
