stdvga: fix screen blanking

stdvga: fix screen blanking

In case the display surface uses a shared buffer (i.e. uses vga vram
directly instead of a shadow) go unshare the buffer before clearing it.

This avoids vga memory corruption, which in turn fixes unblanking not
working properly with X11.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2067
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20240605131444.797896-2-kraxel@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
(cherry picked from commit b1cf266c82cb1211ee2785f1813a6a3f3e693390)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>