Prior to this commit, `sushy-emulator` served all libvirt domains
and OpenStack instances to Redfish clients. This might impose security
risks by exposing too much through Redfish.
This change introduces a way to limit backend VMs exposure to
Redfish clients by way of configuring allowed instance identities
via `SUSHY_EMULATOR_ALLOWED_INSTANCES` configuration option.
Story: 2004305
Task: 27865
Change-Id: Iff194ec63a27300b7687bfcdcb4fca579f111183
7152f1d262