xen: Move xen_setup_callback_vector() definition to include/xen/hvm.h

Kbuild test robot reports the following problem on ARM:

  for 'xen_setup_callback_vector' [-Wmissing-prototypes]
1664 | void xen_setup_callback_vector(void) {}
|      ^~~~~~~~~~~~~~~~~~~~~~~~~

The problem is that xen_setup_callback_vector is a x86 only thing, its
definition is present in arch/x86/xen/xen-ops.h but not on ARM. In
events_base.c there is a stub for !CONFIG_XEN_PVHVM but it is not declared
as 'static'.

On x86 the situation is hardly better: drivers/xen/events/events_base.c
doesn't include 'xen-ops.h' from arch/x86/xen/, it includes its namesake
from include/xen/ which also results in a 'no previous prototype' warning.

Currently, xen_setup_callback_vector() has two call sites: one in
drivers/xen/events_base.c and another in arch/x86/xen/suspend_hvm.c. The
former is placed under #ifdef CONFIG_X86 and the later is only compiled
in when CONFIG_XEN_PVHVM.

Resolve the issue by moving xen_setup_callback_vector() declaration to
arch neutral 'include/xen/hvm.h' as the implementation lives in arch
neutral drivers/xen/events/events_base.c.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lkml.kernel.org/r/20200520161600.361895-1-vkuznets@redhat.com

arch/x86/xen/suspend_hvm.c

@@ -2,6 +2,7 @@
 #include <linux/types.h>
 
 #include <xen/xen.h>
+#include <xen/hvm.h>
 #include <xen/features.h>
 #include <xen/interface/features.h>
 

arch/x86/xen/xen-ops.h

@@ -54,7 +54,6 @@ void xen_enable_sysenter(void);
 void xen_enable_syscall(void);
 void xen_vcpu_restore(void);
 
-void xen_setup_callback_vector(void);
 void xen_hvm_init_shared_info(void);
 void xen_unplug_emulated_devices(void);
 

include/xen/hvm.h

@@ -58,4 +58,6 @@ static inline int hvm_get_parameter(int idx, uint64_t *value)
 #define HVM_CALLBACK_VECTOR(x) (((uint64_t)HVM_CALLBACK_VIA_TYPE_VECTOR)<<\
 		HVM_CALLBACK_VIA_TYPE_SHIFT | (x))
 
+void xen_setup_callback_vector(void);
+
 #endif /* XEN_HVM_H__ */

include/xen/interface/hvm/hvm_op.h

@@ -21,6 +21,8 @@
 #ifndef __XEN_PUBLIC_HVM_HVM_OP_H__
 #define __XEN_PUBLIC_HVM_HVM_OP_H__
 
+#include <xen/interface/xen.h>
+
 /* Get/set subcommands: the second argument of the hypercall is a
  * pointer to a xen_hvm_param struct. */
 #define HVMOP_set_param           0