Since xwayland can only be enabled/disabled at launch, the xwayland
status should be retained on reload. Having `xwayland enabled|disabled`
in the config, should not cause `config->xwayland` to be invalid on
reload. This also returns `CMD_FAILURE` with a message that xwayland
can only be enabled/disabled on launch when trying to set the invalid
status on reload. This allows swaynag to notify the user that the
change will not take effect until sway is restarted.