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

[Richard W.M. Jones]

[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.