VMDK: change get_cluster_offset return type

The return type of get_cluster_offset was an offset that use 0 to denote
'not allocated', this will be no longer true for flat extents, as we see
flat extent file as a single huge cluster whose offset is 0 and length
is the whole file length.
So now we use int return value, 0 means success and otherwise offset
invalid.

Signed-off-by: Fam Zheng <famcool@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
