dom0/core: API CHANGE: pass username as keyword param to vm.run() (#577)

Get rid of ugly embedding username into commandline. This will make much easier
avoid hardcoding username in dom0 code.
Currently dom0 is free of hardcoded "user" username ("root" still is used).

misc/xl-qvm-usb-attach.py

@@ -45,4 +45,4 @@ else:
     qvm_collection.unlock_db()
 
     # launch
-    qvm_collection.get_vm_by_name(backendvm_name).run("root: %s" % cmd)
+    qvm_collection.get_vm_by_name(backendvm_name).run(cmd, user="root")

misc/xl-qvm-usb-detach.py

@@ -41,7 +41,7 @@ else:
     qvm_collection.unlock_db()
 
     # launch
-    qvm_collection.get_vm_by_name(backendvm_name).run("root: %s" % cmd)
+    qvm_collection.get_vm_by_name(backendvm_name).run(cmd, user="root")
 
 # FIXME: command injection
 os.system("xenstore-write /local/domain/%s/backend/vusb/%s/%s/port/%s ''"