xen: free domain's vcpu array

This was overlooked in fb442e2171 ("x86_64: allow more vCPU-s per
guest").

This is XSA-149.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Ian Campbell <ian.campbell@citrix.com>

--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -841,6 +841,7 @@ static void complete_domain_destroy(stru
 
     xsm_free_security_domain(d);
     free_cpumask_var(d->domain_dirty_cpumask);
+    xfree(d->vcpu);
     free_domain_struct(d);
 
     send_global_virq(VIRQ_DOM_EXC);