vmdk: Fix format specific information (create type) for streamOptimized

vmdk: Fix format specific information (create type) for streamOptimized

Previously the field is wrong:

    $ ./qemu-img create -f vmdk -o subformat=streamOptimized /tmp/a.vmdk 1G

    $ ./qemu-img info /tmp/a.vmdk
    image: /tmp/a.vmdk
    file format: vmdk
    virtual size: 1.0G (1073741824 bytes)
    disk size: 12K
    Format specific information:
        cid: 1390460459
        parent cid: 4294967295
>>>     create type: monolithicSparse
        <snip>

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>