[FIX] account_facturx: fix uom unece code fetch

The _get_unece_code method does not work in case where a specific uom
has multiple xml_ids.
Change it to handle such case and try to match every xml ids with a
unece code if possible.

closes odoo/odoo#90939

Signed-off-by: Florian Gilbert <flg@odoo.com>