Mark old VDI.attach as deprecated in docstring
Mark old VDI.attach as deprecated in docstring

I could not add the deprecated annotation as that resulted in a parse
error.

Signed-off-by: Gabor Igloi <7535f5501fd71876c93d896691e178fd3a11cfbb@citrix.com>

xapi-project/message-switch,xapi-project/message-switch,xapi-project/message-switch