Is there a trick to make the last entry in the list of USB host devices visible?
can you scroll down the list with your arrow keys?
FYI virt-manager is not KDE software.
$ rpm -qfi /usr/bin/virt-manager
...
Vendor : Fedora Project
URL : https://virt-manager.org/
Bug URL : https://bugz.fedoraproject.org/virt-manager
...
You can make the windows taller to show more of the list by dragging the bottom edge.
But I think this is worth a bug report against virt-manager to ask why there is a not a scroll bar in that dialog box.
Actually, this is not a bug.
If the list is fully visible vertically, there should not be a vertical scrollbar.
The issue is horizontal.
Widen the window horizontally and the horizontal scrollbar should disappear, and no longer hide the last item in the list.
this is the farthest it goes
I need a bigger screen
or smaller fonts.
But did it work though?
What you describe is a bug. If the scroll bar shortens the windows then the vertical scroll needs to appear.
Not it’s not, most apps do this.
Example:
Window showing a list that has enough screen real estate for the entire list
Window showing a list that does not have enough screen real estate for the entire list. Notice how the horizontal scrollbar shows up because all the text can’t fit.
And an example that is more of a direct comparison to the OP’s issue, but you need to think of my scrollbars as overlay scrollbars, which is what the OP has in virt-manager. Basically, overlay scrollbars show up over the content as opposed to around the content.
If the OP widens the window, the horizontal scrollbar should disappear, allowing full view of the list of items.
Another thing the OP could try, apart from getting a larger screen which costs money, is to use regular scrollbars instead.
If the window loses focus so does the horizontal scrollbar disappear and was I able to find the proper device, it was the last one. Clicking just above the scrollbar did select the device. A bit finicky but possible
that sounds exciting! how do I do that? Couldn’t find it immediately in Systemsettings
Could literally just widen the window but okay. 
I think, iirc, turning off overlay scrollbars is now only possible through qt6ct and/or kvantum. Will check in a bit. Think some themes are like this by default as well.
EDIT
Yes, it’s kvantum for KDE/Qt apps. For GTK apps, it’s either based on the theme (which can be modified) or based on the below environment variable, which sets it for all themes, and is much easier to remember to turn on/off.
Apps using libadwaita and/or GTK 4 might ignore any changes, however.
Example in /etc/environment:
GTK_OVERLAY_SCROLLING=0
Well, I guess you’ll have to file a bug report for the virtualization app, find out how to install kvantum on your distro, or live with it as it is.