Pass the SystemClass as deserialization context for Domains

This allows diminishing the dependency on the parent element when creating a
Domain element from an XML document: the parent was used to get a handle to the
system class in order to check the existence of ConfigurableElements included
in the Domain.

Change-Id: Icba7c3c4db2b9728c0fb7c6840a254de9775f6a4
Signed-off-by: David Wagner <david.wagner@intel.com>