qcow2: Make size_to_clusters() return uint64_t

qcow2: Make size_to_clusters() return uint64_t

Sadly, some images may have more clusters than what can be represented
using a plain int. We should be prepared for that case (in
qcow2_check_refcounts() we actually were trying to catch that case, but
since size_to_clusters() truncated the returned value, that check never
did anything useful).

Cc: qemu-stable <qemu-stable@nongnu.org>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit b6d36def6d9e9fd187327182d0abafc9b7085d8f)

Conflicts:
	block/qcow2-cluster.c
	block/qcow2.h

* removed context dependency on ff99129a
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>