guest: Add USB input devices to pSeries guests with graphics

libvirt already does this for us, sort of: it will automatically
add a USB keyboard and mouse to ppc64 guests with graphics;
in addition, these devices are impossible to remove.

Unfortunately this results in a pretty poor experience for the
user, since the relative pointing device makes interacting with
the GUI an exercise in frustration.

As of commit 186bb479d0f4, libvirt will still add the USB
keyboard automatically but will skip the USB mouse if a USB
tablet is already present, so by explicitly including USB input
devices in the generated XML we can create guests that are
actually usable.

We can do this unconditionally, without having to worry about
what version of libvirt we're running against: if it's new
enough we'll take advantage of the fix, and if not then the
resulting guest will not be any more broken than it would have
been before.

https://bugzilla.redhat.com/show_bug.cgi?id=1683609

Signed-off-by: Andrea Bolognani <abologna@redhat.com>

tests/cli-test-xml/compare/virt-install-ppc64-graphics.xml

@@ -32,6 +32,8 @@
       <source mode="bind"/>
       <target type="virtio" name="org.qemu.guest_agent.0"/>
     </channel>
+    <input type="tablet" bus="usb"/>
+    <input type="keyboard" bus="usb"/>
     <graphics type="vnc" port="-1"/>
     <video>
       <model type="vga"/>

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

@@ -30,6 +30,8 @@
     <channel type="spicevmc">
       <target type="virtio" name="com.redhat.spice.0"/>
     </channel>
+    <input type="tablet" bus="usb"/>
+    <input type="keyboard" bus="usb"/>
     <graphics type="spice" port="-1" tlsPort="-1" autoport="yes">
       <image compression="off"/>
     </graphics>

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

@@ -35,6 +35,8 @@
     <channel type="spicevmc">
       <target type="virtio" name="com.redhat.spice.0"/>
     </channel>
+    <input type="tablet" bus="usb"/>
+    <input type="keyboard" bus="usb"/>
     <graphics type="spice" port="-1" tlsPort="-1" autoport="yes">
       <image compression="off"/>
     </graphics>

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

@@ -32,6 +32,8 @@
       <source mode="bind"/>
       <target type="virtio" name="org.qemu.guest_agent.0"/>
     </channel>
+    <input type="tablet" bus="usb"/>
+    <input type="keyboard" bus="usb"/>
     <graphics type="vnc" port="-1"/>
     <video>
       <model type="vga"/>

virtinst/guest.py

@@ -714,7 +714,7 @@ class Guest(XMLBuilder):
         usb_keyboard = False
         if self.os.is_x86() and not self.os.is_xenpv():
             usb_tablet = self.osinfo.supports_usbtablet()
-        if self.os.is_arm_machvirt():
+        if self.os.is_arm_machvirt() or self.os.is_pseries():
             usb_tablet = True
             usb_keyboard = True