xend: Don't allow migrate with assigned PCI devs.

This can potentially cause a whole machine crash, which I think has
been seen in the wild.  The case in the wild was caused by xendomains
auto-resuming domains on reboot, so it wasn't even operator error.

There's no sane way to migrate or suspend domains with PCI devs,
so I disallow it completely.

Signed-off-by: Mark Williamson <mark.williamson@cl.cam.ac.uk>

tools/python/xen/xend/server/pciif.py

@@ -185,3 +185,6 @@ class PciController(DevController):
 
     def waitForBackend(self,devid):
         return (0, "ok - no hotplug")
+
+    def migrate(self, config, network, dst, step, domName):
+        raise XendError('Migration not permitted with assigned PCI device.')