arm64: KVM: Fix guest dead loop when register accessor returns false

Currently emulate_cp will return 0 (Handled) no matter what the accessor
returns. If register accessor returns false, it will not skip current PC
while emulate_cp return handled. Then guest will stuck in a dead loop.

Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>