qcow2: Fix creation of large images

qcow2: Fix creation of large images

qcow_create2 assumes that the new image will only need one cluster for its
refcount table initially. Obviously that's not true any more when the image is
big enough (exact value depends on the cluster size).

This patch calculates the refcount table size dynamically.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 4768fa902c3860f2fe34403e6e1c83bfca6da034)

Conflicts:

	block/qcow2.c

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