vmdk: Read footer for streamOptimized images

vmdk: Read footer for streamOptimized images

The footer takes precedence over the header when it exists. It contains
the real grain directory offset that is missing in the header. Without
this patch, streamOptimized images with a footer cannot be read.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
(cherry picked from commit 65bd155c7356d448ffee7f89149c4d473076b0ba)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>