desktop: install qemu-guest-agent

Make clipboard and other niceties work out of the box.
Won't run on hardware anyway as it's typically tied to
virtio udev rules.

Author: bluca

mkosi.profiles/desktop/mkosi.conf

@@ -9,6 +9,7 @@ Packages=
         pgpdump
         pipewire
         pipewire-alsa
+        qemu-guest-agent
         wireless-regdb
         xdg-desktop-portal