Not compatible with XQEMU

[JayFoxRox]

There's a number of PGRAPH registers and FIFO behaviour which is probably not emulated in XQEMU (yet).

This is actually an upstream issue, but while the code is still being changed (see #7), there is no point in upstreaming issues.
Supporting XQEMU would have a number of benefits, such as getting a good trace-replay target, more users to create-dumps, and also being able to follow the FIFO logic (assuming it's accurate).