conf: node_device: fix mdev_types format and XML parsing code to match schema

conf: node_device: fix mdev_types format and XML parsing code to match schema

The nodedev schema defines that a mdev_types capability must have
one or more type elements. The XML parsing and the format allows to
accept and to write mdev_types capability without any type element.
This patches fixes this.

Signed-off-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
Reviewed-by: Bjoern Walk <bwalk@linux.ibm.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>