gdbus: fix unexporting interface skeleton when resetting GDBusObjectManager's connection

Submitted by Thomas Haller @thom311

Link to original bug (#757945)

Description

A single interface skelton can be exported via different GDBusObjectManagers. However, resetting the connection of one object-manager would unexport its skeleton from all connections.

This bug effectively makes it impossible to export the same skelton
on multiple object-manager.