use blank storage pool when connecting the volumes

We're not connected to a pool. To prepare an image when you're not
connected to a pool (general HE use case) you should specify a blank
pool uuid (00000000-0000-0000-0000-000000000000)

Change-Id: I3d7683b8dd10097344cb1d5fd036e549fc25ef8c
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1129261
Signed-off-by: Jiri Moskovcak <jmoskovc@redhat.com>