I filed a bug about this at:
https://bugzilla.redhat.com/show_bug.cgi?id=1252780">https://bugzilla.redhat.com/show_bug.cgi?id=1252780
Dropping back to libvirt-1.2.2 fixed it for me, but there was no answer to the bug report.
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[root@localhost ~]# virsh net-start defaulterror: Failed to start network defaulterror: error creating bridge interface virbr0: Operation not supportedsnapshot: