um: xterm: Update options for gnome-terminal

The -x option is deprecated and might be removed in a future release
of gnome-terminal. Let's recommend using -- instead.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20250326070113.401857-3-tiwei.btw@antgroup.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>