xen: compilation fix fo xen CPU hotplugging

This patch fixes compilation error on ia64.
include asm/xen/hypervisor.h instead of asm-x86/xen/hypervisor.h
use xen_pv_domain() instead of is_running_on_xen()

>   CC      drivers/xen/cpu_hotplug.o
> In file included from /linux-2.6/drivers/xen/cpu_hotplug.c:5:
> /linux-2.6/include/asm-x86/xen/hypervisor.h:44:22: error: asm/desc.h: No such file or directory
> /linux-2.6/include/asm-x86/xen/hypervisor.h:66:1: warning: "MULTI_UVMFLAGS_INDEX" redefined
> In file included from /linux-2.6/include/asm-x86/xen/hypervisor.h:52,
>                  from /linux-2.6/drivers/xen/cpu_hotplug.c:5:
> /linux-2.6/arch/ia64/include/asm/xen/hypercall.h:233:1: warning: this is the location of the previous definition
> /linux-2.6/drivers/xen/cpu_hotplug.c: In function 'setup_vcpu_hotplug_event':
> /linux-2.6/drivers/xen/cpu_hotplug.c:81: error: implicit declaration of function 'is_running_on_xen'
> make[4]: *** [drivers/xen/cpu_hotplug.o] Error 1
> make[4]: *** Waiting for unfinished jobs....

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>