cli: Have '--input tablet' default to bus=usb (bz 1232087)

Make the default a little smarter in other ways too

tests/cli-test-xml/compare/virt-install-many-devices.xml

@@ -193,7 +193,7 @@
       <boot order="1"/>
     </interface>
     <input type="keyboard" bus="usb"/>
-    <input type="tablet" bus="xen"/>
+    <input type="tablet" bus="usb"/>
     <graphics type="sdl" display=":3.4" xauth="/tmp/.Xauthority"/>
     <graphics type="spice" port="-1" tlsPort="-1" autoport="yes">
       <image compression="off"/>

tests/clitest.py

@@ -477,8 +477,8 @@ c.add_compare(""" \
 --controller usb,model=ich9-uhci2,address=0:0:4.1,index=0,master=2 \
 --controller usb,model=ich9-uhci3,address=0:0:4.2,index=0,master=4 \
 \
---input keyboard,bus=usb \
---input type=tablet \
+--input type=keyboard,bus=usb \
+--input tablet \
 \
 --serial tcp,host=:2222,mode=bind,protocol=telnet \
 --parallel udp,host=0.0.0.0:1234,bind_host=127.0.0.1:1234 \

virtinst/deviceinput.py

@@ -39,8 +39,15 @@ class VirtualInputDevice(VirtualDevice):
     type = XMLProperty("./@type",
                        default_cb=lambda s: s.TYPE_MOUSE,
                        default_name=TYPE_DEFAULT)
+
+    def _default_bus(self):
+        if self.type == self.TYPE_TABLET:
+            return self.BUS_USB
+        if self.conn.is_xen():
+            return self.BUS_XEN
+        return self.BUS_PS2
     bus = XMLProperty("./@bus",
-                      default_cb=lambda s: s.BUS_XEN,
+                      default_cb=_default_bus,
                       default_name=BUS_DEFAULT)