- Edited
The partition manager in the Calamares installer was so easy to use, compared to GEdit or KDE Partition Manager that I don't know why I had trouble with it when I created the Plasma VM - just infamiliarity and a rush to get something done, I guess. I went slower this time, and had no problems creating what I needed, including a swap space big enough for hibernating the finished VM.
Unfortunately, the installation failed when it came time to install the boot loader. I created a bug report in the Dev Tracker. To see whether it was a problem with the partition manager instead of the installer, I'm going to try once more, this time using GParted to prepare the .VDI before attemptin the installation. If that succeeds, I won't log another bug report, but I'll return to here to explain.
When I started GParted, the .VDI disk looked perfect. It included all the correct flags, including the ESP flag along with the BOOT flag, although that hadn't been visible in the Calamares partition manager. But I didn't want to attempt another installation with the same file, and expect a different result, so I created a new partition table and started over from the beginning.
This time the installation continued on to completion. I amended the bug report appropriatey. And indeed, after removing the .ISO file from the virtual CD drive, the device launches just fine. I'm a bit confused by the UI as it appears here, compared with the upper bar and dock that my previous Xfce VMs had, but I suspect it was done this way to appeal to those who don't prefer the GNOME look. I approve. It would also be nice to get rid of that wallpaper before this is released, I think.
I'll continue to configure this VM and report any problems I find. And I'll leave the arrangement of the UI the same, since users probably will as well.