qemu-nbd: don't overlap long option values with short options

When defining values for long options, the normal practice is
to start numbering from 256, to avoid overlap with the range
of valid values for short options.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
