codegen, sdk: Rename "export" to "export_"

The classes generated by the generateDS.py tool contain an "export"
method intended to convert the objects to the corresponding XML
document. The broker classes generated by our own code generator also
generate "export" methods for some resources that have an "export"
action, for example virtual machines.  These two methods clash, making
it impossible to generate XML for some broker classes. To avoid this
class this patch modifies our code generator so that it will modify the
code generated with generateDS.py, renaming the "export" method to
"export_".

Change-Id: Iad8a031684ddaed11c1b11efc379e746888cd0a2
Bug-Url: https://bugzilla.redhat.com/1167851
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>