ui+display: rename is_buffer_shared() -> surface_is_allocated()

ui+display: rename is_buffer_shared() -> surface_is_allocated()

Boolean return value is reversed, to align with QEMU_ALLOCATED_FLAG, so
all callers must be adapted.  Also rename share_surface variable in
vga_draw_graphic() to reduce confusion.

No functional change.

Suggested-by: Marc-André Lureau <marcandre.lureau@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20240605131444.797896-4-kraxel@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>