virtio-scsi by default on power

Power architecture guests should default to virtio-scsi rather than spapr-scsi

Signed-off-by: Scott Garfinkle <scottgar@linux.vnet.ibm.com>

tests/cli-test-xml/compare/virt-install-ppc64-pseries-f20.xml

@@ -20,6 +20,7 @@
       <target dev="sda" bus="scsi"/>
       <readonly/>
     </disk>
+    <controller type="scsi" index="0" model="virtio-scsi"/>
     <graphics type="spice" port="-1" tlsPort="-1" autoport="yes">
       <image compression="off"/>
     </graphics>

virtinst/guest.py

@@ -1012,7 +1012,7 @@ class Guest(XMLBuilder):
                     break
 
         # Add virtio-scsi controller if needed
-        if (self.os.is_arm_machvirt() and
+        if ((self.os.is_arm_machvirt() or self.os.is_pseries()) and
             not has_any_scsi and
             not has_virtio_scsi):
             for dev in self.get_devices("disk"):