LUSetClusterParms: fix validation of beparams

Since the contents of the dict is validated via the ForceDictType, we can
simply require that it is a dict here. The previous check was wrong, as it was
copied from the HV checks (which also doesn't verify the leaf dict type).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>