virtinst: Use virtio bus type for disks on pSeries machines

pSeries doesn't support ide and so was changed to use the scsi. virtio works
better on pSeries with better performance outcomes. Change the default to
virtio for disks

Signed-off-by: Shivaprasad G Bhat <sbhat@linux.vnet.ibm.com>

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

@@ -17,7 +17,7 @@
     <disk type="file" device="disk">
       <driver name="qemu" type="qcow2"/>
       <source file="/dev/default-pool/testvol1.img"/>
-      <target dev="sda" bus="scsi"/>
+      <target dev="vda" bus="virtio"/>
     </disk>
     <graphics type="spice" port="-1" tlsPort="-1" autoport="yes">
       <image compression="off"/>

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

@@ -17,7 +17,7 @@
     <disk type="file" device="disk">
       <driver name="qemu" type="qcow2"/>
       <source file="/dev/default-pool/testvol1.img"/>
-      <target dev="sda" bus="scsi"/>
+      <target dev="vda" bus="virtio"/>
     </disk>
     <graphics type="spice" port="-1" tlsPort="-1" autoport="yes">
       <image compression="off"/>

tests/cli-test-xml/compare/virt-install-ppc64le-kvm-import.xml

@@ -17,7 +17,7 @@
     <disk type="file" device="disk">
       <driver name="qemu" type="qcow2"/>
       <source file="/dev/default-pool/testvol1.img"/>
-      <target dev="sda" bus="scsi"/>
+      <target dev="vda" bus="virtio"/>
     </disk>
     <interface type="bridge">
       <source bridge="eth0"/>

virtinst/guest.py

@@ -1036,7 +1036,7 @@ class Guest(XMLBuilder):
                     self._os_object.supports_virtiodisk()))):
                 d.bus = "virtio"
             elif self.os.is_pseries():
-                d.bus = "scsi"
+                d.bus = "virtio"
             elif self.os.is_arm():
                 d.bus = "sd"
             elif self.os.is_q35():