OptsVisitor, StringInputVisitor and the keyval visitor have
three different ideas of how a human could write the value of
a boolean option.  Pay homage to the backwards-compatibility
gods and make the new common helper accept all four sets (on/off,
true/false, y/n and yes/no), but remove case-insensitivity.

Since OptsVisitor is supposed to match qemu-options, adjust
it as well.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201103161339.447118-1-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>