The version 2 of the mouse position message is using the (channel_id,
monitor_id) pair as the unique identifier for the monitor. The pair is
then translated to the guest_output_id (called display_id in the
vd_agent message) either by retrieving it from the
monitors_guest_output_id hash table, if it is in there, or the old way:
channel_id ? channel_id : monitor_id.

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