hw/display: fix virgl reset regression

hw/display: fix virgl reset regression

Before commit 49afbca3b00e8e517d54964229a794b51768deaf ("virtio-gpu: drop
use_virgl_renderer"), use_virgl_renderer was preventing calling GL
functions from non-GL context threads. The innocuously looking

  g->parent_obj.use_virgl_renderer = false;

was set the first time virtio_gpu_gl_reset() was called, during
pc_machine_reset() in the main thread. Further virtio_gpu_gl_reset()
calls in IO threads, without associated GL context, were thus skipping
GL calls and avoided warnings or crashes (see also
https://gitlab.freedesktop.org/virgl/virglrenderer/-/issues/226).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210702123221.942432-1-marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>