I'm on a Fedora 20 host and I installed a Fedora 20 VM using virt-manager. When I check the Display > "Auto resize VM with window", it worked as exepcted. Then I updated the VM to Fedora 21 and the auto resize feature doesn't work any more. I tried running spice-vdagent in debug mode inside the guest. On the F20 guest, I can see log messages when I resize the window. However, there is no message when I resize the F21 guest. Clipboard syncing still works on the F21 guest. I can see the issue with virt-manager or virt-viewer. I see nothing interesting in the spice-vdagentd journal except "closed/opening vdagent virtio channel" messages.
I am running 64-bit virt-viewer 1.0 for Windows and my Fedora 21 VM (with GNOME) is affected. My Fedora 20 VM (with GNOME) is not affected. Host is RHEL 7 + oVirt 3.5.0. Both VMs are set to SPICE, not VNC.
After installing a bunch of host OS updates and guest OS updates, I am no longer affected on my above-mentioned configuration.