Test_vdi_cbt: move to the Opt module in stdext
Test_vdi_cbt: move to the Opt module in stdext

To facilitate the planned transition to an upstream option combinator
module - we can simply look for uses of stdext's Opt module if we use it
everywhere it's needed, instead of reimplementing the needed function.

Signed-off-by: Iglói Gábor <73b73dea312d5bc8698ae242a300b452b938c56b@gmail.com>

srowe/xen-api,srowe/xen-api,srowe/xen-api,srowe/xen-api,srowe/xen-api