Make force_migrate not replace existing node

Previous to this, if once force-migrated a VM, the previous_node value
would be updated to the current node, which is likely never what an
administrator would want. Change this functionality so that the previous
node value is not changed, and update the documentation to reflect this.

client-cli/pvc.py

@@ -563,7 +563,7 @@ def vm_move(domain, target_node, selector):
 )
 @click.option(
     '-f', '--force', 'force_migrate', is_flag=True, default=False,
-    help='Force migrate an already migrated VM.'
+    help='Force migrate an already migrated VM; does not replace an existing previous node value.'
 )
 def vm_migrate(domain, target_node, selector, force_migrate):
     """