'List volumes on the backend available for management by Cinder.
Returns a list of dictionaries, each specifying a volume in the host,
with the following keys:
- reference (dictionary): The reference for a volume, which can be
passed to "manage_existing".
- size (int): The size of the volume according to the storage
backend, rounded up to the nearest GB.
- safe_to_manage (boolean): Whether or not this volume is safe to
manage according to the storage backend. For example, is the volume
in use or invalid for any reason.
- reason_not_safe (string): If safe_to_manage is False, the reason why.
- cinder_id (string): If already managed, provide the Cinder ID.
- extra_info (string): Any extra information to return to the user
:param cinder_volumes: A list of volumes in this host that Cinder
currently manages, used to determine if
a volume is manageable or not.
:param marker:    The last item of the previous page; we return the
next results after this value (after sorting)
:param limit:     Maximum number of items to return
:param offset:    Number of items to skip after marker
:param sort_keys: List of keys to sort results by (valid keys are
\'identifier\' and \'size\')
:param sort_dirs: List of directions to sort by, corresponding to
sort_keys (valid directions are \'asc\' and \'desc\')'

def get_manageable_volumes(self, cinder_volumes, marker, limit, offset, sort_keys, sort_dirs):

    return []
