powerpc/vio: Use device_type to detect family

Currently in the vio.c code we use a comparision against the parent
device node's full path to decide if the device is a PFO or VIO family
device.

Both the ibm,platform-facilities and vdevice nodes are defined by PAPR,
and must have a matching device_type. So instead of using the path we
can instead compare the device_type.

I've checked Qemu and kvmtool both do this correctly, and all the
PowerVM systems I have access to do also. So it seems to be safe.

This removes the dependency on full_name, which is being removed
upstream.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>