When I use virt-manager to change which virtual network a guest's NIC is attached to, it tells me I need to restart the guest. This is a bit odd since I can remove a virtio NIC and add a replacement (on the new network) without restarting. Why can't you just indicate a loss of carrier, then the carrier returns with a connection to the new network? I appreciate Windows is crap but "you have plugged you Ethernet cable into a new network; you now must reboot" was really only a myth, surely?
Looks like libvirt has API support for this nowadays, it's just not wired up in virt-manager. I'll poke at it.
Fixed upstream now
virt-manager-1.0.1-1.fc20 has been submitted as an update for Fedora 20. https://admin.fedoraproject.org/updates/virt-manager-1.0.1-1.fc20
Package virt-manager-1.0.1-1.fc20: * should fix your issue, * was pushed to the Fedora 20 testing repository, * should be available at your local mirror within two days. Update it with: # su -c 'yum update --enablerepo=updates-testing virt-manager-1.0.1-1.fc20' as soon as you are able to. Please go to the following url: https://admin.fedoraproject.org/updates/FEDORA-2014-4273/virt-manager-1.0.1-1.fc20 then log in and leave karma (feedback).
virt-manager-1.0.1-1.fc20 has been pushed to the Fedora 20 stable repository. If problems still persist, please make note of it in this bug report.