Fixed.

I had installed a preview repository from Red Hat for some not-yet-released libguestfs features (libguestfs-RHEL-7.1-preview). There were evidently changes to some release packages in the updates repository that were incompatible with the candidate packages in the preview repository cited above.

I disabled the preview repo in /etc/yum.repos.d/…, uninstalled each of its packages, and re-installed the relevant packages (libvirt qemu–kvm qemu–kvm-tools virt-* spice-gtk spice-gtk-python spice-gtk-tools spice-gtk3-vala spice-xpi).

Lesson learned: When you install 3rd party repositories you should reserve time for hand-holding your package management system.