xen: Move manage.c to drivers/xen for ia64/xen support

move arch/x86/xen/manage.c under drivers/xen/to share codes
with x86 and ia64.
ia64/xen also uses manage.c

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>