qemu: use long on|off syntax for -spice boolean option values

qemu: use long on|off syntax for -spice 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 -spice since at least 1.5.3,
so we don't need to check for it.

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