virt-sandbox-image: fix error string formatting.
virt-sandbox-image: fix error string formatting.

libvirt/libvirt-sandbox,agx/libvirt-sandbox-debian,agx/libvirt-sandbox-debian,libvirt/libvirt-sandbox,agx/libvirt-sandbox-debian,libvirt/libvirt-sandbox,agx/libvirt-sandbox-debian,agx/libvirt-sandbox-debian