vmdk: Fix next_cluster_sector for compressed write

vmdk: Fix next_cluster_sector for compressed write

When the VMDK is streamOptimized (or compressed), the
next_cluster_sector must not be incremented by a fixed number of
sectors. Instead of this, it must be rounded up to the next consecutive
sector. Fixing this results in much smaller compressed images.

Signed-off-by: Radoslav Gerganov <rgerganov@vmware.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>