CA-95928: Xapi lists and attempts to mount directories as isos

Signed-off-by: Sugandha Aggarwal <sugandha.aggarwal@citrix.com>
Acked-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Vineeth Raveendran <vineeth.thampi@citrix.com>
Imported-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #130 on xapi-project/sm