[Libguestfs] [PATCH libnbd 0/2] copy: Set default request-size to 2**18 (262144 bytes)

As Nir has pointed out, our current default for nbdcopy --request-size is far from optimal. In this patch series I have changed the default to something which is better, and provided some benchmark results. With this simplistic approach it's not possible to choose a default which is best in all situations. That will likely require us to benchmark many machines and try to work out a formula relating measurable aspects of those machines like L3 cache size to the best request size, but that's a lot more work. Also one of the tests implicitly depended on the default size so I had to adjust the test. Rich.