qapi: Lift features into QAPISchemaEntity

qapi: Lift features into QAPISchemaEntity

Commit 6a8c0b5102 "qapi: Add feature flags to struct types" added
features to QAPISchemaObjectType.  Commit a95daa5093 "qapi: Add
feature flags to commands in qapi" added them to QAPISchemaCommand,
duplicating the code.  Tolerable, but the duplication will only get
worse as we add features to more definitions.

To de-duplicate, lift features from QAPISchemaObjectType and
QAPISchemaCommand into QAPISchemaEntity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191024110237.30963-18-armbru@redhat.com>