I've had the exact same problem and I finally managed to solve it:
My issue was that the X-server crashed because of the `vmmouse` driver. To fix this I've followed this post https://forum.manjaro.org/t/qemu-vm-no-x-after-stable-update/93601/2 .
1. Boot into the Manjaro Live ISO
Edit your VM, go into XML View (top right corner) and change the "<boot order='2'/>" of the cdrom device to 1 and the disk device to 2.
After you've done that, start the VM and it should boot into the Manjaro Live ISO.
2. Open terminal and chroot into your manjaro installation:
Follow the chroot section of https://wiki.archlinux.org/title/chroot , to get the path to the drive just use the file manager.
3. Remove the "xf86-input-vmmouse" package
Run
$ sudo pacman -Rns xf86-input-vmmouse
in the chroot'ed terminal.
Now, change to boot order back and boot into the Manjaro installation and it should work again.
Other tips for newbies like myself
To get ssh access to the vm with no gui: Follow steps 1 and 2, and enable the ssh daemon with "sudo systemctl enable sshd". Then boot back into the installation and start a ssh session on a different machine to get full terminal access.
See what crashed with "journalctl -r".
