guest: Default to e1000e if machine=q35

And libosinfo doesn't have any specific recommendation for us

tests/cli-test-xml/compare/virt-install-boot-uefi.xml

@@ -40,6 +40,7 @@
     <interface type="bridge">
       <source bridge="eth0"/>
       <mac address="00:11:22:33:44:55"/>
+      <model type="e1000e"/>
     </interface>
     <console type="pty"/>
     <channel type="spicevmc">

tests/cli-test-xml/compare/virt-install-q35-defaults.xml

@@ -53,6 +53,7 @@
     <interface type="bridge">
       <source bridge="eth0"/>
       <mac address="00:11:22:33:44:55"/>
+      <model type="e1000e"/>
     </interface>
     <console type="pty"/>
     <channel type="spicevmc">
@@ -120,6 +121,7 @@
     <interface type="bridge">
       <source bridge="eth0"/>
       <mac address="00:11:22:33:44:55"/>
+      <model type="e1000e"/>
     </interface>
     <console type="pty"/>
     <channel type="spicevmc">

virtinst/guest.py

@@ -1107,6 +1107,9 @@ class Guest(XMLBuilder):
         else:
             net_model = self._os_object.default_netmodel()
 
+        if not net_model and self.os.is_q35():
+            net_model = "e1000e"
+
         if net_model:
             for net in self.devices.interface:
                 if not net.model: