Starting GNOME Boxes and trying to launch my Windows XP VM, I get a generic error about the VM not being able to be started. On the terminal, I get this instead: (gnome-boxes:30174): Boxes-WARNING **: libvirt-broker.vala:47: Failed to update domain 'Windows XP.iso': Failed to set domain configuration: unsupported configuration: QEMU 2.1.2 is too new for help parsing (gnome-boxes:30174): Boxes-WARNING **: machine.vala:576: Failed to start Windows XP Pro SP3: Unable to start domain: unsupported configuration: QEMU 2.1.2 is too new for help parsing (gnome-boxes:30174): Boxes-WARNING **: machine.vala:576: Failed to start Windows XP Pro SP3: Unable to start domain: unsupported configuration: QEMU 2.1.2 is too new for help parsing
https://www.redhat.com/archives/libvir-list/2014-December/msg00304.html suggests this error may actually indicate something else wrong (it seems this 'help parsing' only happens when something else has already gone wrong), FWIW.
Moving to POST to backport the error reporting fix at least
libvirt-1.2.9.3-1.fc21 has been submitted as an update for Fedora 21. https://admin.fedoraproject.org/updates/libvirt-1.2.9.3-1.fc21
Package libvirt-1.2.9.3-1.fc21: * should fix your issue, * was pushed to the Fedora 21 testing repository, * should be available at your local mirror within two days. Update it with: # su -c 'yum update --enablerepo=updates-testing libvirt-1.2.9.3-1.fc21' as soon as you are able to. Please go to the following url: https://admin.fedoraproject.org/updates/FEDORA-2015-7150/libvirt-1.2.9.3-1.fc21 then log in and leave karma (feedback).