Add a display_id field to the structure that we use to send down the
list of guest display resolutions to the vdagentd daemon. This allows us
to map the spice display id to the proper X display for determining
mouse locations, etc.

In the case where we have an mjpeg plugin running in the streaming
agent, we have two spice displays representing the same guest display.
In that scenario, the session agent may maintain the following guest
output mapping:
 spice channel 0 (QXL) => X output 0
 spice channel 1 (streaming-agent) => X output 0

While this is not necessarily a supported scenario, it would be nice if
the cursor input worked properly in this case. The root problem is that
when the session agent sends down the guest xorg resolutions to the
system daemon, it simply loops through the list of xorg displays, and
for each X display it looks up the first spice display ID associated
with it and sends that down to the daemon. In the scenario mentioned
above, since ...