xenapi: ensure vdi is not too big when resizing down

Fixes bug 1155066

This check ensures that when a user chooses to resize thier
instance to a smaller size and it is not possible, their
VM is still running, but will be left in the Error state.

Ideally we can report this back to the user without it going
to the Error state, this will be a future change.

Change-Id: Ibad568ab3cfb9caaf4fe002572c8cda973d501a7