xen/x86: move x86-only variable out of common

Commit 27c36a12f1 is an x86-only feature.  As such xen_evtchn_needs_ack
should only exist on x86.

Differential Revision: https://reviews.freebsd.org/D29913
Reviewed by: royger
[royger]: adjust comment.

sys/x86/include/xen/xen-os.h

@@ -43,6 +43,9 @@
 /* If non-zero, the hypervisor has been configured to use a direct vector */
 extern int xen_vector_callback_enabled;
 
+/* Signal whether the event channel vector requires EOI at the lapic */
+extern bool xen_evtchn_needs_ack;
+
 /* tunable for disabling PV disks */
 extern int xen_disable_pv_disks;
 

sys/xen/hvm.h

@@ -102,6 +102,5 @@ void xen_hvm_suspend(void);
 void xen_hvm_resume(bool suspend_cancelled);
 
 extern uint32_t hvm_start_flags;
-extern bool xen_evtchn_needs_ack;
 
 #endif	/* __XEN_HVM_H__ */