nxos_user: fails to remove usernames with embedded '\' (#53149)

Example: username ucs-DOMAIN\\x password 0 foo

Found by `common/sanity` test.

lib/ansible/modules/network/nxos/nxos_user.py

@@ -360,6 +360,7 @@ def main():
         have_users = [x['name'] for x in have]
         for item in set(have_users).difference(want_users):
             if item != 'admin':
+                item = item.replace("\\", "\\\\")
                 commands.append('no username %s' % item)
 
     result['commands'] = commands