Prevent corruption of VM in VirtualBox when using linked clone

Fix https://github.com/GNS3/gns3-gui/issues/1821

gns3server/compute/virtualbox/virtualbox_vm.py

@@ -603,9 +603,16 @@ class VirtualBoxVM(BaseNode):
         :param vmname: VirtualBox VM name
         """
 
+        if vmname == self._vmname:
+            return
+
         if self.linked_clone:
             if self.status == "started":
                 raise VirtualBoxError("You can't change the name of running VM {}".format(self._name))
+            # We can't rename a VM to name that already exists
+            vms = yield from self.manager.list_vms(allow_clone=True)
+            if vmname in [vm["vmname"] for vm in vms]:
+                raise VirtualBoxError("You can't change the name to {} it's already use in VirtualBox".format(vmname))
             yield from self._modify_vm('--name "{}"'.format(vmname))
 
         log.info("VirtualBox VM '{name}' [{id}] has set the VM name to '{vmname}'".format(name=self.name, id=self.id, vmname=vmname))