In particular in the PV case xzalloc_bytes() forcing SMP_CACHE_BYTES
alignment is counterproductive, as the allocation size there is only 40
bytes.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/irq.c
+++ b/xen/arch/x86/irq.c
@@ -1313,9 +1313,12 @@ void cleanup_domain_irq_mapping(struct d
 
 struct pirq *alloc_pirq_struct(struct domain *d)
 {
-    size_t sz = is_hvm_domain(d) ? sizeof(struct pirq) :
-                                   offsetof(struct pirq, arch.hvm);
-    struct pirq *pirq = xzalloc_bytes(sz);
+    union pirq_pv {
+        char space[offsetof(struct pirq, arch.hvm)];
+        void *align;
+    };
+    struct pirq *pirq = is_hvm_domain(d) ? xzalloc(struct pirq)
+                                         : (void *)xzalloc(union pirq_pv);
 
     if ( pirq )
     {