dom0: when cleaning up network devices, wait for actual device destroy

Otherwise subsequent network-attach will not be noticed by frontend driver.

dom0/qvm-core/qubes.py

@@ -609,6 +609,15 @@ class QubesVm(object):
                     # Don't check retcode - it always will fail when backend domain is down
                     subprocess.call(["/usr/sbin/xl",
                             "network-detach", self.name, m.group(1)], stderr=subprocess.PIPE)
+                    # Wait for device destroy
+                    tries = 0
+                    path = "'{0}/device/vif/0/state".format(xs.get_domain_path(self.xid))
+                    while xs.read(path):
+                        time.sleep(0.1)
+                        tries += 1
+                        if tries > 10:
+                            # timeout
+                            break
 
     def create_xenstore_entries(self, xid):
         if dry_run: