[Libguestfs] [PATCH nbdkit] sparse-random: Don't generate random content in blocks by default

[I already pushed this upstream, this email is FYI] As discussed earlier today, when testing nbdcopy with nbdkit- sparse-random-plugin as the test harness, a very large amount of time was spent generating random numbers to fill the data blocks. This was pointless make-work, and this patch fixes it. More details in the commit message. Rich.