kvmtool: ARM64: Add target type potenza for aarch64

The VCPU target type KVM_ARM_TARGET_XGENE_POTENZA is available
in latest Linux-3.16-rcX or higher hence register aarch64 target
type for it.

This patch enables us to run KVMTOOL on X-Gene Potenza host.

Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Signed-off-by: Anup Patel <anup.patel@linaro.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
[will: reworded comment for clarity]
Signed-off-by: Will Deacon <will.deacon@arm.com>