Fix QEMU domain save to block devices with cgroups enabled

Fix QEMU domain save to block devices with cgroups enabled

When cgroups is enabled, access to block devices is likely to be
restricted to a whitelist. Prior to saving a guest to a block device,
it is necessary to add the block device to the whitelist. This is
not required upon restore, since QEMU reads from stdin

* src/qemu/qemu_driver.c: Add block device to cgroups whitelist
  if neccessary during domain save.