vm/qubesvm: forbid changing VM name

We've decided to make VM name immutable. This is especially important
for Admin API, where some parts (especially policy) are sticked to the
VM name.

Now, to rename the VM, one need to clone it under new name (thanks to
LVM, this is very quick action), then remove the old one.

Fixes QubesOS/qubes-issues#2868

qubes/vm/qubesvm.py

@@ -372,7 +372,7 @@ class QubesVM(qubes.vm.mix.net.NetVMMixin, qubes.vm.BaseVM):
         doc='''Internal, persistent identificator of particular domain. Note
             this is different from Xen domid.''')
 
-    name = qubes.property('name', type=str,
+    name = qubes.property('name', type=str, write_once=True,
         clone=False,
         doc='User-specified name of the domain.')