Fix handling of disk backing stores with cgroups

Fix handling of disk backing stores with cgroups

The cgroups ACL code was only allowing the primary disk image.
It is possible to chain images together, so we need to search
for backing stores and add them to the ACL too. Since the ACL
only handles block devices, we ignore the EINVAL we get from
plain files. In addition it was missing code to teardown the
cgroup when hot-unplugging a disk

* src/qemu/qemu_driver.c: Allow backing stores in cgroup ACLs
  and add missing teardown code in unplug path