drm/virtio: Add missing virtqueue reset

As per the VirtIO spec, the virtqueues must be reset during cleanup
(see "3.3.1 Driver Requirements: Device Cleanup").

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190102175507.4653-2-ezequiel@collabora.com