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

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.