fix tgt-admin to create sheepdog target

In original implementation, if we've set targets.conf to create targets with Sheepdog, we always saw the error messages such as "# unix:/var/lib/sheepdog/sock:hoge does not exist - please check the configuration file".

This is due to checking the existence of backing-store's file, except RBD.
If we want to use Sheepdog as backing-store, we'll set the untypical string(this is NOT filesystem's path) to the backing-store's path such as "unix:path_of_unix_domain_socket:vdi" or "tcp:host:port:vdi". therefore the backing-store's file is always not exist on filesystem.

This patch works to skip checking the existence of backing-store's file if bs-type is sheepdog. thereby we were succeed in creating targets with Sheepdog.