Sync fs and move image deletion after loop detach

The warden tests frequently fail under VMware due to an error detaching a loop device that was associated with a mounted disk image:
```
  loop: can't delete device /dev/loop0: Device or resource busy
```
In order to reduce the likelihood of the problem, a `sync` was added before the detach and the deletion of the image file was moved until after the detach.  After repeated runs with these modifications, I have not seen the failures occur.