[Libguestfs] [PATCH nbdkit] luks: Don't advertise zero support

[Richard W.M. Jones]

Eric: This seems like a bug in nbdkit, but this patch just does the
easy fix.  What do you think?

Two other filters can return NBDKIT_ZERO_EMULATE:

filters/ext2/ext2.c:  return NBDKIT_ZERO_EMULATE;
filters/nozero/nozero.c:    return NBDKIT_ZERO_EMULATE;

nbdkit-ext2-filter doesn't crash, but it doesn't emulate either and it
seems broken.  It calls down to the underlying plugin (which will
either emulate or true zero).  I believe it may be zeroing a random
offset in the underlying disk.

Rich.