os-win should silently pass when requested to disconnect a vhd that's missing.
At the moment, it's erroring out, which prevents some Nova instances from being
cleaned up.
This change checks if the image to be disconnected actually exists before
attempting to open it.
Change-Id: Ic734d38e1939165a89a79842d37e98b4cbbcb3bf
Closes-Bug: #1810927