image-fuzzer: Encode file name and file format to bytes

image-fuzzer: Encode file name and file format to bytes

Callers of create_image() will pass strings as arguments, but the
Image class will expect bytes objects to be provided.  Encode
them inside create_image().

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20191016192430.25098-9-ehabkost@redhat.com
Message-Id: <20191016192430.25098-9-ehabkost@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>