qemu: use on|off for -vnc boolean option values

qemu: use on|off for -vnc boolean option values

The preferred syntax for boolean options is to set the value "on" or
"off". QEMU 7.1.0 will deprecate the short format we currently use.

The long format has been supported with -vnc since the change to use
QemuOpts in 2.2.0, so we check based on the new capability flag.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>