spice: allow to specify drm rendernode

When multiple GPU are available, picking the first one isn't always the
best choice. Learn to specify a device rendernode.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20170212112118.16044-1-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>