fix compilation error of ballon driver on ia64.
extent_start member is pointer argument. On x86 pointer argument for
xen hypercall is passed as virtual address.
On the other hand, ia64 and ppc, pointer argument is passed in pseudo
physical address. (guest physicall address.)
So they must be passed as handle and convert right before issuing hypercall.

  CC      drivers/xen/balloon.o
linux-2.6-x86/drivers/xen/balloon.c: In function 'increase_reservation':
linux-2.6-x86/drivers/xen/balloon.c:228: error: incompatible types in assignment
linux-2.6-x86/drivers/xen/balloon.c: In function 'decrease_reservation':
linux-2.6-x86/drivers/xen/balloon.c:324: error: incompatible types in assignment
linux-2.6-x86/drivers/xen/balloon.c: In function 'dealloc_pte_fn':
linux-2.6-x86/drivers/xen/balloon.c:486: error: incompatible types in assignment
linux-2.6-x86/drivers/xen/balloon.c: In function 'alloc_empty_pages_and_pagevec':
linux-2.6-x86/drivers/xen/balloon.c:522: error: incompatible types in assignment
make[2]: *** [drivers/xen/balloon.o] Error 1

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>