nbd-server-add: Fix the default for 'writable'

nbd-server-add: Fix the default for 'writable'

The documentation to this monitor command tells, that 'writable'
argument is optional and defaults to false. However, the code sets
true as the default. But since some applications may already been
using this, it's safer to fix the code and not documentation which
would break those applications.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>