GNOME Boxes edit configuration feature does not work

Impact
======
GNOME Boxes has a feature to edit the libvirt xml configuration file to allow for additional configuration changes beyond those includes in the Preferences dialog.

Due to a bug, changes were not being saved and used by Boxes.

Test Case
=========
1. From Ubuntu 23.04, install GNOME Boxes
2. Open GNOME Boxes
3. If you do not already have an existing box set up:
   a. Click the + button to create a new box using an existing downloaded ISO or let it download one for you.
   b. The New Virtual Machine dialog will display (after it finishes downloading the ISO if needed)
   c. The Create button should be active
   d. Click the Create button and Boxes should start a virtual machine and boot that ISO
   e. Complete the install
4. From the main Boxes screen, right click on the box and select Preferences
5. Switch to the Devices & Shares page and make sure that there is an ISO set up for CDROM/DVD Drive (if not, click + and select an existing bootable ISO like an Ubuntu Installer ISO)
6. Switch back to the Resources page. Click Edit Configuration. It will ask if you want to create a snapshot; either answer is fine.
7. Your text editor will open with an xml file. Find the line that says
   <boot dev="hd"/>
8. Change "hd" in that line to "cdrom"
9. Save the file. Close the file.
10. Click the box to start it. It should boot the ISO instead of the hard drive.

What Could Go Wrong
===================
As a component of GNOME core, there is a micro-release exception for GNOME Boxes.
https://wiki.ubuntu.com/StableReleaseUpdates/GNOME

This fix will likely be included in GNOME Boxes 44.1 due in a few days.

This is a one-line patch.