Add virtio-gpu.rst to the corresponding section in MAINTAINERS, so that
the maintainers gets CC:-ed on corresponding patches.

Message-ID: <20231027060808.242442-1-thuth@redhat.com>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>