The name of the second parameter differs between QAPI schema and C
implementation: it's @protocol in the former and @file in the latter.
Potentially confusing.  Change the C implementation to match the QAPI
schema.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20231031104531.3169721-2-armbru@redhat.com>