docs/interop: add "firmware.json"

docs/interop: add "firmware.json"

Add a schema that describes the different uses and properties of virtual
machine firmware.

Each firmware executable installed on a host system should come with at
least one JSON file that conforms to this schema. Each file informs the
management applications about
- the firmware's properties and one possible use case / feature set,
- configuration bits that are required to run the firmware binary.

In addition, define rules for management apps for picking the highest
priority firmware JSON file when multiple such files match the search
criteria.

Cc: "Daniel P. Berrange" <berrange@redhat.com>
Cc: David Gibson <dgibson@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Kashyap Chamarthy <kchamart@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20180509152608.9343-1-lersek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>