I got a Widnows VM with Looking Glass working by following the ArchWiki and the installation steps at the Looking Glass website. Looking Glass is working fine for the most part, however the copy-and-paste functionality only works when copying from the real PC to the VM, not the other way around.
Current setup:
Host: ArchLinux with Kernel 5.3.7 and Plasma 5.17 as DE
Guest OS: Windows 10 May 2019
Setup a qemu channel using unix sockets and a spice channel using spicevmc
Disabled QXL adapter by setting “model type=‘none’”
So far my tests include:
Making sure the Looking Glass host and viewer versions match (I’m using B1 version)
Installing (and reinstalling) VirtIO drivers and Spice Guest tools including version 0.1-161
Making sure the qemu-ga and spice-vda services are running on Windows
Testing multiple combinations of spice:clipboard, spice:clipboardToVM and spice:clipboardToLocal
Disabling Plasma clipboard manager and restarting Plasma
Thank you both for your support, unfortunately reinstalling Spice nor using Ctrl + Q worked. However the issue seems to be limited to Windows 10 1903, as I was able to get it working right on Windows 10 1809.