Comment 6 for bug 1393842

Same issue here. syslog shows that app-armor is refusing the creation of the socket:

Dec 29 10:07:09: kernel: [ 1957.839479] audit: type=1400 audit(1419876429.922:90): apparmor="DENIED" operation="mknod" profi
le="libvirt-85c4cb3e-a2a1-42ba-af30-5d2d8f989780" name="/var/lib/libvirt/qemu/channel/target/CentOS-7.org.qemu.guest_agent.0" pid=7861 comm
="qemu-system-x86" requested_mask="c" denied_mask="c" fsuid=116 ouid=116

Following the steps described above by Mark Grocock fixed the issue.

You need to be careful when editing the app-armor config since the last line of the file closes the 'profile qemu_bridge_helper' group. The added line must be after the curly braces. I have also restarted app-armor, I don't know if it was required.