Implement VDI.{attach,detach}
Implement VDI.{attach,detach}

In attach, we look for any existing loopdevs, and return any we find.
In detach, we look for any existing loopdevs, and detach one if it exists.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

franciozzy/ffs,djs55/ffs,xapi-project/ffs,robertbreker/ffs