CA-120469: Fail gracefully in case rmdir returns ENOTEMPTY

Parallel vm-suspend operations on vm that has got vdi's in the same SR
could result in reference files, ignoring rmdir error (ENOTEMPTY)
instead of lsdir looks a better option.

Signed-off-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>
Reviewed-by:Germano Percossi <germano.percossi@citrix.com>

GitHub: closes #53 on xapi-project/sm