backend: ExportRepoImageParameters default constructor

This fixes a weird serialization/deserialization issue that resulted
in destinationDomainId to be null.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=987986
Change-Id: I63556883c8d81ce3914feac5a76d102d3964c678
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>