qom: make user_creatable_complete() specific to UserCreatable

Instead of accepting any Object*, change user_creatable_complete() to
require a UserCreatable*. Modify the callers to pass the appropriate
argument, removing redundant dynamic cast checks in object creation.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20181204142023.15982-4-marcandre.lureau@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>