x86/MSI: adjust permitted vector range

It's really FIRST_IRQ_VECTOR vector which is meant here, i.e. effectively
there is a form of open-coding in use right now.

No change in practice, due to the present aliasing.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>

xen/arch/x86/msi.c

@@ -158,7 +158,7 @@ void msi_compose_msg(unsigned vector, const cpumask_t *cpu_mask, struct msi_msg
 {
     memset(msg, 0, sizeof(*msg));
 
-    if ( vector < FIRST_DYNAMIC_VECTOR )
+    if ( vector < FIRST_IRQ_VECTOR )
         return;
 
     if ( cpu_mask )