qemu-options: update to show preferred boolean syntax for -netdev

The preferred syntax is to use "foo=on|off", rather than a bare
"foo" or "nofoo".

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210216191027.595031-5-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>