virtio-gpu: track and limit host memory allocations

This patch makes virtio-gpu track host memory allocations for ressources
and applies a limit (configurable 256M by default).  When exceeding the
limit virtio-gpu throws VIRTIO_GPU_RESP_ERR_OUT_OF_MEMORY errors (like
it already does today when pixman image allocations fail).

This patch covers 2d mode only.  For 3d mode we have to figure how we
are going to handle this best.  qemu doesn't track resources in case
virglrenderer is used, so I guess we should extend virglrenderer to
allow setting a limit, then let qemu set the limit and catch
virgl_renderer_resource_create failures.

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: 李强 <liqiang6-s@360.cn>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1480423356-22255-1-git-send-email-kraxel@redhat.com
