tests: benchmark crypto with fixed data size, not time period

tests: benchmark crypto with fixed data size, not time period

Currently the crypto benchmarks are processing data in varying chunk
sizes, over a fixed time period. This turns out to be a terrible idea
because with small chunk sizes the overhead of checking the elapsed
time on each loop iteration masks the true performance.

Benchmarking over a fixed data size avoids the loop running any system
calls which can interfere with the performance measurements.

Before this change

Enc chunk 512 bytes 2283.47 MB/sec Dec chunk 512 bytes 2236.23 MB/sec OK
Enc chunk 4096 bytes 2744.97 MB/sec Dec chunk 4096 bytes 2614.71 MB/sec OK
Enc chunk 16384 bytes 2777.53 MB/sec Dec chunk 16384 bytes 2678.44 MB/sec OK
Enc chunk 65536 bytes 2809.34 MB/sec Dec chunk 65536 bytes 2699.47 MB/sec OK

After this change

Enc chunk 512 bytes 2058.22 MB/sec Dec chunk 512 bytes 2030.11 MB/sec OK
Enc chunk 4096 bytes 2699.27 MB/sec Dec chunk 4096 bytes 2573.78 MB/sec OK
Enc chunk 16384 bytes 2748.52 MB/sec Dec chunk 16384 bytes 2653.76 MB/sec OK
Enc chunk 65536 bytes 2814.08 MB/sec Dec chunk 65536 bytes 2712.74 MB/sec OK

The actual crypto performance hasn't changed, which shows how
significant the mis-measurement has been for small data sizes.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>