pflash_cfi01: Remove user_creatable flag

TYPE_CFI_PFLASH01 devices need to be mapped by
pflash_cfi01_register() (or equivalent) and can't be used with
-device. Remove user_creatable from the device class.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: qemu-block@nongnu.org
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170503203604.31462-7-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>