xen: yield to IPI target if necessary

When sending a call-function IPI to a vcpu, yield if the vcpu isn't
running.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>

diff --git a/arch/x86/xen/xen-ops.h b/arch/x86/xen/xen-ops.h
index b5697ba..3847eed 100644
--- a/arch/x86/xen/xen-ops.h
+++ b/arch/x86/xen/xen-ops.h
@@ -27,6 +27,8 @@
 int xen_set_wallclock(unsigned long time);
 unsigned long long xen_sched_clock(void);
 
+bool xen_vcpu_stolen(int vcpu);
+
 void xen_mark_init_mm_pinned(void);
 
 void __init xen_fill_possible_map(void);