restapi: Move JAXB global bindings to external file

These bindings aren't part of the specification of the API, but of the
implementation, so there is no reason to show them to the users. In
addition they cause issues in the generator of the Java SDK, as it tries
to use type converters that aren't available there.

Change-Id: I0c79ae190d3c811c7d225ff5a293e14c541e688d
Bug-Url: https://bugzilla.redhat.com/1063443
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>