To keep compatibility with old endpoints (any of client, server,
vd_agent), we need to copy the message to add the output_id field.

The output_id is the guest-side id of the xrandr output (to be precise,
it is the index in the list of xrandr outputs) that is set in the
monitors config messages by the streaming agent. It is later used in the
guest by vd_agent for mouse input and possibly monitors_config
(enabling/disabling monitors and setting the resolution/position of
monitors).

Signed-off-by: Lukáš Hrázký <lhrazky@redhat.com>