virgl/vtest: add support to vtest for new cap getting.

virgl/vtest: add support to vtest for new cap getting.

The vtest protocol is pretty simple but also pretty dumb, and
the v1 caps query was fixed size, with no nice way to expand it,
however the server also ignores any command it doesn't understand.

So we can query v2 caps by sending a v2 followed by a v1, if the
v2 is ignored we know it's an old vtest server, and the we get
a v2 answer then we can just read the v1 answer and discard it.

Acked-by: Jakob Bornecrantz <jakob@collabora.com> (sounds good)