Problem
When running Fedora 43 Beta Workstation in a KVM (Boxes, Cockpit, virt-manager…) virtual machine, features that depend on the spice-vdagent guest agent - like copy/paste between the host and the guest - do not work.
Cause
This happens because the guest agent does not start automatically, which is meant to happen when the system detects that it is running as a KVM guest (and which happened correctly in Fedora 42 and earlier releases).
Related Issues
Bugzilla report: #2394505
Latest upstream fix: MR #54
Workarounds
You can start the service manually by running systemctl --user start spice-vdagent as your regular user (not as root). This should immediately cause things to start working as normal.