ui/console: Fix console resize with placeholder surface

ui/console: Fix console resize with placeholder surface

In `qemu_console_resize()`, the old surface of the console is keeped if the new
console size is the same as the old one. If the old surface is a placeholder,
and the new size of console is the same as the placeholder surface (640*480),
the surface won't be replace.
In this situation, the surface's `QEMU_PLACEHOLDER_FLAG` flag is still set, so
the console won't be displayed in SDL display mode.
This patch fixes this problem by forcing a new surface if the old one is a
placeholder.

Signed-off-by: Tianlan Zhou <bobby825@126.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20240207172024.8-1-bobby825@126.com>
(cherry picked from commit 95b08fee8f68d284a5028d37fd28be7a70c8e92b)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>