vmdk: fix VMFS extent parsing

The VMFS extent line in description file doesn't have start offset as
FLAT lines does, and it should be defaulted to 0. The flat_offset
variable is initialized to -1, so we need to set it in this case.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
