vmdk: Fix converting to streamOptimized

vmdk: Fix converting to streamOptimized

Commit d62d9dc4b8 lifted streamOptimized images's version to 3, but we
now refuse to open version 3 images read-write.  We need to make
streamOptimized an exception to allow converting to it. This fixes the
accidentally broken iotests case 059 for the same reason.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit 3db1d98a20262228373bb973ca62b1ab64b29af4)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>