gallium/virgl: Add code path for virgl to read driconf

gallium/virgl: Add code path for virgl to read driconf

This works only for the drm variant of virgl and not for the vtest
variant.

v2: Rebase, replace the configuration query function by a pointer to
    the configuration data.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com> (v1)
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>