qcow2: Avoid overflow in alloc_clusters_noref()

alloc_clusters_noref() stores the cluster index in a uint64_t. However,
offsets are often represented as int64_t (as for example the return
value of alloc_clusters_noref() itself demonstrates). Therefore, we
should make sure all offsets in the allocated range of clusters are
representable using int64_t without overflows.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
