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Virtual monitor heads are not "connected" until viewed in a front end
Description of problem:
When you attach a virtual GPU to a guest, qemu appears to only "attach" a virtual monitor to an output port when that virtual display is
viewed using the GUI. For example, when you boot using the above command line, there will be four displays in ```/sys/class/drm/``` on the guest,
```card0-Virtual-1``` through to ```card0-Virtual-4```. In each of these directories, there is an "enabled" file, which contains either
"enabled" or "disabled". These contain "disabled" until you switch tab/view to look at it using the GUI, at which point they change to "enabled".
This causes a problem for us because Weston will not initialise displays that do not have a monitor attached, meaning the system we are trying
to boot fails because not all the Weston display surfaces are available.
There does not appear to be a command line option to force virtual monitors to be attached to virtual displays immediately. Looking through the
Gtk user interface code (and the other front ends) there does not appear to be a call into the qemu core that requests the connection of a virtual
monitor to the virtual displays - my guess is that qemu only connects a monitor when a render request first happens (or similar), but I have not followed the code paths deeper than the source files in ```QEMU/ui/```.
I also tried using the ```screengrab``` command to screenshot each head, but this does not need sufficient to cause the display to be marked
enabled in the guest.
While we could possibly automate the GUI using some external tool, we ultimately need to run this in a CI environment using
```egl-headless``` or similar.
Steps to reproduce:
1. Launch qemu with virtio-gpu-gl setting max_outputs > 1
2. On guest, ```cat /sys/drm/class/card0-Virtual-2``` - it reads "disabled"
3. On host, switch the view to look at the second display ("virtio-gpu-gl-pci.1")
4. On guest, ```cat /sys/drm/class/card0-Virtual-2``` - it now reads "enabled"
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