Our testsuite coverage of nbd_get_block_size() is pretty sparse (only
the recent commit 6f5fec2ea uses them in errors-server-unaligned.c for
debug purposes, and even that requires recent patches in nbdkit).  But
in the process of adding an interop test with qemu-nbd, I also noticed
that qemu-nbd (at least version 6.2) has client-dependent behavior:
fails NBD_OPT_INFO for older clients that don't request block size,
and fudges the value to 1 for NBD_OPT_GO for back-compat reasons.  We
still want to request by default, but now we need a knob, similar to
the existing set_full_info(), for overriding our defaults for testing
purposes.

Message-Id: <20220224172620.1701230-1-eblake@redhat.com>
Acked-by: Richard W.M. Jones <rjones@redhat.com>