I installed "virt-manager" 2.2.1 on my 20.04 desktop and created a VM in it. I find that many of the settings require clicking the "light bulb" on the toolbar - but mine doesn't have that!
Did I miss installing a component I need?
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The light bulb button (or the "Show hardware information" button) can be found is the virtual machine console's toolbar. You first need to open the console by double clicking on the virtual machine or by using the "Open" button.
If you use a custom GTK theme you may have an issue and the light bulb will be invisible but the button will still be working.
Here are some screenshots to illustrate. The first one displays the main window and the second one the console window with the light bulb button: