Don't copy USB bus/address by default from node device

Putting vendor+product AND bus+addr in the XML is only needed if the
user has duplicate USB devices attached to the machine. If we always
pass bus + addr, then we've regressed for the vast majority of users
who don't care about duplicate devices.

Author: crobinso

tests/nodedev-xml/devxml/usbdev1.xml

@@ -2,6 +2,5 @@
       <source>
         <vendor id='0x0781'/>
         <product id='0x5151'/>
-        <address bus='1' device='4'/>
       </source>
     </hostdev>

virtinst/VirtualHostDevice.py

@@ -214,8 +214,10 @@ def _set_from_nodedev(self, nodedev):
 
         self.vendor = nodedev.vendor_id
         self.product = nodedev.product_id
-        self.bus = nodedev.bus
-        self.device = nodedev.device
+
+        if not (self.vendor or self.product):
+            self.bus = nodedev.bus
+            self.device = nodedev.device
 
     def _get_source_xml(self):
         xml = ""