I have a few .img files in a directory. When I double click on them it shows a dialog that the file type is unknown. I expect that either virt-manager would launch or it would tell me that I need to install it. Perhaps .img is not registered as a mime type with a handler. And probably virt-manager isn't registering itself as being able to launch those type of files.
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Moving to rawhide
Moving to upstream tracker
Boxes handles this kind of desktop integration these days, so just punting to that