hw/display/virtio-gpu: Improve "opengl is not available" error message

If the user tries to use the virtio-gpu-gl device but the display
backend doesn't have OpenGL support enabled, we currently print a
rather uninformative error message:

$ qemu-system-aarch64 -M virt -device virtio-gpu-gl
qemu-system-aarch64: -device virtio-gpu-gl: opengl is not available

Since OpenGL is not enabled on display frontends by default, users
are quite likely to run into this. Improve the error message to
be more specific and to suggest to the user a path forward.

Note that the case of "user tried to enable OpenGL but the display
backend doesn't handle it" is caught elsewhere first, so we can
assume that isn't the problem:

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

(Use of error_append_hint() requires us to add an ERRP_GUARD() to
the function, as noted in include/qapi/error.h.)

With this commit we now produce the hopefully more helpful error:
$ ./build/x86/qemu-system-aarch64 -M virt -device virtio-gpu-gl
qemu-system-aarch64: -device virtio-gpu-gl: The display backend does not have OpenGL support enabled
It can be enabled with '-display BACKEND,gl=on' where BACKEND is the name of the display backend to use.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2443
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>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-ID: <20240731154136.3494621-2-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

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