virgl: Negotiate version with vtest server

Check if server supports version negotation by sending a PING_PROTOCOL_VERSION
message right before a dummy RESOURCE_BUSY_WAIT. If we don't get a reply
for the first, we know the server doesn't support it.

If it does support it, we can query the max protocol version supported
by the server and fall back if needed.

v2: - Send a new message to negotiate the protocol version, checking if
      the server supports this message by immediately sending a busy wait
      message. (Dave Airlie)

v3: - Send a zero-arg command PING_PROTOCOL_VERSION so we actually keep
      compatibility with older servers. (Code by Dave Airlie)

Signed-off-by: Tomeu Vizoso <[email protected]>
Reviewed-by: Gurchetan Singh <[email protected]>

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