domain: cpu: Clear 'migratable' and 'check' when changing mode

Otherwise going host-passthrough -> host-model -> host-passthrough
can cause libvirt validation errors or leave unintended bits in
the XML

Signed-off-by: Cole Robinson <crobinso@redhat.com>

virtinst/domain/cpu.py

@@ -286,6 +286,8 @@ class DomainCpu(XMLBuilder):
             self.model = None
             self.vendor = None
             self.model_fallback = None
+            self.migratable = None
+            self.check = None
             for f in self.features:
                 self.remove_child(f)
             self.mode = val