virgl: Re-use and extend queue transfers for intersecting buffer subdatas.

Small buffer subdatas which are essentially doing a memcpy were getting
bogged down by all the overhead of creating new transfers.

Signed-off-by: David Riley <davidriley@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
14 jobs for !613 with extend-transfer6 in 10 minutes and 23 seconds (queued for 11 seconds)
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