Often you want to transfer clipboard text between host and guest. It would be handy if this could be done with virt-viewer. For maximum safety, it could present a dialog prompt on demand, where you paste the text targeted for the guest. Once you've reviewed that the pasted text is actually what you intended to pass on, then you press OK and virt-viewer puts the text in the guest clipboard buffer.
Similarily in reverse, the dialog would display the guest buffer, and to put it in host buffer you would need to select and copy.
GTK-VNC / VNC already have support for cut+paste, but there's no support for this in QEMU's VNC server, hence it is pointless trying to enable it in virt-viewer currently. We can't do anything further until QEMU supports this.
If the guest is configured to have SPICE graphics, and has a SPICE virtio serial channel configured, and the SPICE guest agent installed, cut+paste does now work with virt-viewer.
There's still nothing we can do about VNC yet
Realistically I don't think this will ever be done for VNC, so I'm going to close this