cpu: Drop vendor and features if setting host-model (bz 1190851)

Otherwise via virt-manager we can try to set a <cpu> with a <vendor>
but no <model>, and libvirt complains.

tests/xmlparse-xml/change-minimal-guest-out.xml

@@ -23,7 +23,7 @@
     <label>frob</label>
   </seclabel>
   <cpu mode="custom" match="exact">
-    <model>foobar</model>
+    <model fallback="allow">foobar</model>
     <topology sockets="1" cores="4" threads="1"/>
     <feature policy="forbid" name="x2apic"/>
   </cpu>

tests/xmlparse.py

@@ -251,6 +251,7 @@ class XMLParseTest(unittest.TestCase):
 
         check = self._make_checker(guest.cpu)
         check("model", None, "foobar")
+        check("model_fallback", None, "allow")
         check("cores", None, 4)
         guest.cpu.add_feature("x2apic", "forbid")
         guest.cpu.set_topology_defaults(guest.vcpus)

virtinst/cpu.py

@@ -59,6 +59,10 @@ class CPU(XMLBuilder):
         if (val == self.SPECIAL_MODE_HOST_MODEL or
             val == self.SPECIAL_MODE_HOST_PASSTHROUGH):
             self.model = None
+            self.vendor = None
+            self.model_fallback = None
+            for f in self.features:
+                self.remove_feature(f)
             self.mode = val
         elif val == self.SPECIAL_MODE_HOST_COPY:
             self.copy_host_cpu()
@@ -161,6 +165,7 @@ class CPU(XMLBuilder):
                 self.match = "exact"
         return val
     model = XMLProperty("./model", set_converter=_set_model)
+    model_fallback = XMLProperty("./model/@fallback")
 
     match = XMLProperty("./@match")
     vendor = XMLProperty("./vendor")