With 1.4.2 the code no longer checks for xenpv when adding
a usb tablet which is only supported on xen hvm.

--- virt-manager-1.4.2/virtinst/guest.py.orig	2017-08-09 11:47:19.812789816 -0600
+++ virt-manager-1.4.2/virtinst/guest.py	2017-08-09 11:49:36.023630599 -0600
@@ -659,7 +659,7 @@ class Guest(XMLBuilder):
 
         usb_tablet = False
         usb_keyboard = False
-        if self.os.is_x86():
+        if self.os.is_x86() and not self.os.is_xenpv():
             usb_tablet = self._os_object.supports_usbtablet()
         if self.os.is_arm_machvirt():
             usb_tablet = True