vmdk: Only read cid from image file when opening

Previously cid of parent is parsed from image file for every IO request.
We already have L1/L2 cache and don't have assumption that parent image
can be updated behind us, so remove this to get more efficiency.

The parent CID is checked only for once after opening.

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