ia64/pv_ops/xen: elf note based xen startup.

This patch enables elf note based xen startup for IA-64, which gives the
kernel an early hint for running on xen like x86 case.
In order to avoid the multi entry point, presumably extending booting
protocol(i.e. extending struct ia64_boot_param) would be necessary.
It probably means that elilo also needs modification.

Signed-off-by: Qing He <qing.he@intel.com>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Tony Luck <tony.luck@intel.com>