um: xterm: Add Wayland support

Under Wayland, we should check WAYLAND_DISPLAY instead.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20250326070113.401857-2-tiwei.btw@antgroup.com


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