qcow2: initialize metadata before inserting in cluster_allocs

QCow2Meta structure was inserted into list before many fields are
initialized. Currently is not a problem cause all occur in a lock
but if qcow2_alloc_clusters would in a future unlock this lock
some issues could arise.
Initializing fields before inserting fix the problem.

Signed-off-by: Frediano Ziglio <freddy77@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
