Upgraded from rc3 to rc4 and VM's now not loading. Moved back to rc3 and issue disappears. Let me know if there are any othert tests I can do.
Relevant part (I think) of the log file:
Jun 7 10:00:21 T emhttp: Starting libvirt...
Jun 7 10:00:21 T logger: Starting libvirtd...
Jun 7 10:00:21 T kernel: tun: Universal TUN/TAP device driver, 1.6
Jun 7 10:00:21 T kernel: tun: © 1999-2004 Max Krasnyansky
Jun 7 10:00:22 T kernel: Ebtables v2.0 registered
Jun 7 10:00:22 T kernel: device virbr0-nic entered promiscuous mode
Jun 7 10:00:22 T avahi-daemon[10153]: Joining mDNS multicast group on interface virbr0.IPv4 with address 192.168.122.1.
Jun 7 10:00:22 T avahi-daemon[10153]: New relevant interface virbr0.IPv4 for mDNS.
Jun 7 10:00:22 T avahi-daemon[10153]: Registering new address record for 192.168.122.1 on virbr0.IPv4.
Jun 7 10:00:22 T kernel: virbr0: port 1(virbr0-nic) entered listening state
Jun 7 10:00:22 T kernel: virbr0: port 1(virbr0-nic) entered listening state
Jun 7 10:00:22 T dnsmasq[10776]: started, version 2.57 cachesize 150
Jun 7 10:00:22 T dnsmasq[10776]: compile time options: IPv6 GNU-getopt no-DBus I18N DHCP TFTP IDN
Jun 7 10:00:22 T dnsmasq-dhcp[10776]: DHCP, IP range 192.168.122.2 -- 192.168.122.254, lease time 1h
Jun 7 10:00:22 T dnsmasq[10776]: reading /etc/resolv.conf
Jun 7 10:00:22 T dnsmasq[10776]: using nameserver 192.168.1.1#53
Jun 7 10:00:22 T dnsmasq[10776]: read /etc/hosts - 1 addresses
Jun 7 10:00:22 T dnsmasq[10776]: read /var/lib/libvirt/dnsmasq/default.addnhosts - 0 addresses
Jun 7 10:00:22 T dnsmasq-dhcp[10776]: read /var/lib/libvirt/dnsmasq/default.hostsfile
Jun 7 10:00:22 T kernel: virbr0: port 1(virbr0-nic) entered disabled state
Jun 7 10:00:23 T ntpd[1492]: Listen normally on 4 virbr0 192.168.122.1:123
Jun 7 10:00:23 T ntpd[1492]: new interface(s) found: waking up resolver
Jun 7 10:00:33 T kernel: docker0: port 1(veth220da9e) entered forwarding state
Jun 7 10:00:33 T kernel: docker0: port 2(veth046e65d) entered forwarding state
Jun 7 10:00:34 T kernel: docker0: port 3(vethbe3047d) entered forwarding state
Jun 7 10:00:34 T kernel: br0: topology change detected, propagating
Jun 7 10:00:34 T kernel: br0: port 1(eth0) entered forwarding state
Jun 7 10:00:34 T kernel: docker0: port 4(vethcdf1f8c) entered forwarding state
Jun 7 10:00:45 T kernel: kvm: already loaded the other module
Jun 7 10:00:45 T kernel: php[11589]: segfault at 0 ip 00002b71b16540fa sp 00007ffdafd9b290 error 4 in libvirt-php.so[2b71b1642000+36000]