cli: Don't hardcode nec-xhci for --controller usb3

Use our helper function to choose the optimal model

Author: crobinso

tests/cli-test-xml/compare/virt-install-singleton-config-2.xml

@@ -129,7 +129,7 @@
   </pm>
   <devices>
     <emulator>/new/emu</emulator>
-    <controller type="usb" index="0" model="nec-xhci"/>
+    <controller type="usb" index="0" model="qemu-xhci" ports="8"/>
     <controller type="virtio-scsi" index="0"/>
     <filesystem type="mount" accessmode="mapped">
       <source dir="/foo/source"/>
@@ -317,7 +317,7 @@
   </pm>
   <devices>
     <emulator>/new/emu</emulator>
-    <controller type="usb" index="0" model="nec-xhci"/>
+    <controller type="usb" index="0" model="qemu-xhci" ports="8"/>
     <controller type="virtio-scsi" index="0"/>
     <filesystem type="mount" accessmode="mapped">
       <source dir="/foo/source"/>

virtinst/cli.py

@@ -2346,9 +2346,7 @@ def _parse(self, inst):
         if self.optstr == "usb2":
             return DeviceController.get_usb2_controllers(inst.conn)
         elif self.optstr == "usb3":
-            inst.type = "usb"
-            inst.model = "nec-xhci"
-            return inst
+            return DeviceController.get_usb3_controller(inst.conn, self.guest)
         return VirtCLIParser._parse(self, inst)