'Return spec for creating disk-less backing.
:param name: Name of the backing
:param ds_name: Datastore name where the disk is to be provisioned
:param profileId: Storage profile ID for the backing
:param extra_config: Key-value pairs to be written to backing\'s
extra-config
:return: Spec for creation'

def _get_create_spec_disk_less(self, name, ds_name, profileId=None, extra_config=None):

    cf = self._session.vim.client.factory
    vm_file_info = cf.create('ns0:VirtualMachineFileInfo')
    vm_file_info.vmPathName = ('[%s]' % ds_name)
    create_spec = cf.create('ns0:VirtualMachineConfigSpec')
    create_spec.name = name
    create_spec.guestId = 'otherGuest'
    create_spec.numCPUs = 1
    create_spec.memoryMB = 128
    create_spec.files = vm_file_info
    create_spec.version = 'vmx-08'
    if profileId:
        vmProfile = cf.create('ns0:VirtualMachineDefinedProfileSpec')
        vmProfile.profileId = profileId
        create_spec.vmProfile = [vmProfile]
    if extra_config:
        if (BACKING_UUID_KEY in extra_config):
            create_spec.instanceUuid = extra_config.pop(BACKING_UUID_KEY)
        create_spec.extraConfig = self._get_extra_config_option_values(extra_config)
    return create_spec
