qcow2: Return 0/-errno in qcow2_alloc_cluster_offset Returning 0/-errno allows it to distingush different errors classes. The cluster offset of newly allocated clusters is now returned in the QCowL2Meta struct. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>