virgl: Support copy transfers

virgl: Support copy transfers

Support transfers that use a different resource as the source of data to
transfer. This will be used in upcoming commits to send data to host
buffers through a transfer upload buffer, in order to avoid waiting
when the buffer resource is busy.

Note that we don't support queueing copy transfers in the transfer
queue. Copy transfers should be emitted directly in the command queue,
allowing us to avoid flushes before them and leads to better
performance.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>