ui/console: Add placeholder flag to message surface

ui/console: Add placeholder flag to message surface

The surfaces created with former qemu_create_message_surface
did not display the content from the guest and always contained
simple messages describing the reason.

A display backend may want to hide the window showing such a
surface. This change renames the function to
qemu_create_placeholder_surface, and adds "placeholder" flag; the
display can check the flag to decide to do anything special like
hiding the window.

Signed-off-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Message-Id: <20210225101316.83940-1-akihiko.odaki@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>