vmx: support outputing the type attribute for MAC addresses

vmx: support outputing the type attribute for MAC addresses

When support for MAC addresses having a type='static|generated'
attribute was added in:

  commit 454e5961abf40c14f8b6d7ee216229e68fd170bf
  Author: Bastien Orivel <bastien.orivel@diateam.net>
  Date:   Mon Jul 13 16:28:53 2020 +0200

    Add a type attribute on the mac address element

the VMX -> XML parser was not updated. As a result while we
accept the 'type' attribute on input, we never show it again
on 'output', so we loose information during the roundtrip.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>