Extract and specialize domain import- and export-specific contexts

Import and Export of ConfigurableDomains have different context needs: e.g. the
value representation is only used for export; auto-validation is only
meaningful for import whereas the "with settings" context is common to both.

We create two new classes, derived from XmlDomainSerializingContext and move
most of its content to each class it belongs to.

Change-Id: I56589cdb3a8ea417e11d2ed98ccd055d7cdead67
Signed-off-by: David Wagner <david.wagner@intel.com>