Description of problem:

Upon VM start, libvirt uses automatic address allocation for defining addresses   for each device (when not defined addresses explicitly).
As part of the automatic allocation, libvirt limits maximum units (disks)
for each controller [6 (narrow bus) / 15 (wide bus)].

In order to bypass this limitation, engine should set each device address explicitly (keeping unit value unique and consecutive in the VM devices).

More details and full discussion:
https://www.redhat.com/archives/libvir-list/2013-November/msg01113.html