This is an apparmor issue. Apparmor denies attaching the snapshotted volume to a guest, example log output from manual runs of the given test show:

Jul 14 13:33:35 manualnovaci kernel: [ 7317.047425] type=1400 audit(1468503215.581:148): apparmor="STATUS" operation="profile_replace" profile="unconfined" name="libvirt-7cf6b321-0188-4a3c-8e2a-cda7f944405f" pid
=31697 comm="apparmor_parser"
Jul 14 13:33:35 manualnovaci kernel: [ 7317.269093] type=1400 audit(1468503215.805:149): apparmor="DENIED" operation="open" profile="libvirt-7cf6b321-0188-4a3c-8e2a-cda7f944405f" name="/mnt/quobyte-volume/abfa10
02557ab2b21ec218a86487dd92/volume-c26c8016-2e42-48bf-a112-f14c62d48f9a.6fb6e32e-6a2f-47ba-b9d3-a36d55c21db3" pid=31439 comm="qemu-system-x86" requested_mask="r" denied_mask="r" fsuid=1000 ouid=1000
Jul 14 13:33:35 manualnovaci kernel: [ 7317.269112] type=1400 audit(1468503215.805:150): apparmor="DENIED" operation="open" profile="libvirt-7cf6b321-0188-4a3c-8e2a-cda7f944405f" name="/mnt/quobyte-volume/abfa10
02557ab2b21ec218a86487dd92/volume-c26c8016-2e42-48bf-a112-f14c62d48f9a.6fb6e32e-6a2f-47ba-b9d3-a36d55c21db3" pid=31439 comm="qemu-system-x86" requested_mask="r" denied_mask="r" fsuid=1000 ouid=1000
Jul 14 13:33:35 manualnovaci kernel: [ 7317.269138] type=1400 audit(1468503215.805:151): apparmor="DENIED" operation="open" profile="libvirt-7cf6b321-0188-4a3c-8e2a-cda7f944405f" name="/mnt/quobyte-volume/abfa10
02557ab2b21ec218a86487dd92/volume-c26c8016-2e42-48bf-a112-f14c62d48f9a.6fb6e32e-6a2f-47ba-b9d3-a36d55c21db3" pid=31439 comm="qemu-system-x86" requested_mask="r" denied_mask="r" fsuid=1000 ouid=1000
Jul 14 13:33:36 manualnovaci kernel: [ 7318.080239] type=1400 audit(1468503216.617:152): apparmor="STATUS" operation="profile_replace" profile="unconfined" name="libvirt-211d6603-8366-4e0f-9a89-44a23704597a" pid
=31707 comm="apparmor_parser"
Jul 14 13:33:36 manualnovaci kernel: [ 7318.333090] type=1400 audit(1468503216.869:153): apparmor="DENIED" operation="open" profile="libvirt-211d6603-8366-4e0f-9a89-44a23704597a" name="/mnt/quobyte-volume/abfa10
02557ab2b21ec218a86487dd92/volume-d08277bd-1380-49fe-a94e-c4ba38f5c976.4966661d-4491-4ea9-aa25-30b9d2aa4cf5" pid=31476 comm="qemu-system-x86" requested_mask="r" denied_mask="r" fsuid=1000 ouid=1000
Jul 14 13:33:36 manualnovaci kernel: [ 7318.333106] type=1400 audit(1468503216.869:154): apparmor="DENIED" operation="open" profile="libvirt-211d6603-8366-4e0f-9a89-44a23704597a" name="/mnt/quobyte-volume/abfa10
02557ab2b21ec218a86487dd92/volume-d08277bd-1380-49fe-a94e-c4ba38f5c976.4966661d-4491-4ea9-aa25-30b9d2aa4cf5" pid=31476 comm="qemu-system-x86" requested_mask="r" denied_mask="r" fsuid=1000 ouid=1000
Jul 14 13:33:36 manualnovaci kernel: [ 7318.333131] type=1400 audit(1468503216.869:155): apparmor="DENIED" operation="open" profile="libvirt-211d6603-8366-4e0f-9a89-44a23704597a" name="/mnt/quobyte-volume/abfa10
02557ab2b21ec218a86487dd92/volume-d08277bd-1380-49fe-a94e-c4ba38f5c976.4966661d-4491-4ea9-aa25-30b9d2aa4cf5" pid=31476 comm="qemu-system-x86" requested_mask="r" denied_mask="r" fsuid=1000 ouid=1000
Jul 14 13:33:37 manualnovaci kernel: [ 7318.840828] type=1400 audit(1468503217.377:156): apparmor="STATUS" operation="profile_replace" profile="unconfined" name="libvirt-7cf6b321-0188-4a3c-8e2a-cda7f944405f" pid
=31709 comm="apparmor_parser"