vmdk: fix buf leak in vmdk_parse_extents()

vmdk_open_sparse() does not take ownership of buf so the caller always
needs to free it.

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