context-windows: ability to assign a disk to a certain letter in Windows

[OpenNebulaSupport]

Some customers need to have either of these two options:

• assign context image to specified letter (e.g. attach context ISO as Z drive) or;
• assign some image (not the context one) to specified letter (e.g. disk with data to drive D).

It looks like TARGET attribute could be a good candidate for that but it seems one needs to implement additional checking for the value of that attribute since the linux device has three letters but windows one is just single letter.

According to that Stackoverflow post the following command can be used to change drive letter:

Get-Partition -DriveLetter D | Set-Partition -NewDriveLetter E