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

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

The on|off syntax has been supported since -vnc switched to use
QemuOpts in commit 4db14629

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210216191027.595031-7-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>