xen: fix boolean parameter handling

xen: fix boolean parameter handling

Commit 63e8a1e5ffa7a7fdbde887805f673fea7e8d2e94 ("xen: check parameter
validity when parsing command line") introduced a bug for the case
when a boolean parameter was specified by its keyword only (no value).
It would set just the wrong boolean value for that parameter.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>