Display VM screenshot

[david-crespo]

In addition to full VNC support, we plan to have an endpoint where you can get a screenshot of the current state of the desktop on a VM. We will want to display that somewhere, maybe several somewheres. @pfmooney pictured it as a sort of thumbnail preview you could use to identify or disambiguate instances. It could also be used for some very minor debugging. See oxidecomputer/propolis#610