system/vl.c: Expand OpenGL related errors

system/vl.c: Expand OpenGL related errors

Expand the OpenGL related error messages we produce for various
"OpenGL not present/not supported" cases, to hopefully guide the
user towards how to fix things.

Now if the user tries to enable GL on a backend that doesn't
support it the error message is a bit more precise:

$ qemu-system-aarch64 -M virt -device virtio-gpu-gl -display curses,gl=on
qemu-system-aarch64: OpenGL is not supported by display backend 'curses'

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
[AJB: Improved error report message]
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-ID: <20240731154136.3494621-3-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>