hw: Simplify accesses to the CPUState::'start-powered-off' property

hw: Simplify accesses to the CPUState::'start-powered-off' property

The 'start-powered-off' property has been added to ARM CPUs in
commit 5de164304a ("arm: Allow secondary KVM CPUs to be booted
via PSCI"), then eventually got generalized to all CPUs in commit
c1b701587e ("target/arm: Move start-powered-off property to generic
CPUState"). Since all CPUs have it, no need to check whether it is
available. Updating this property can't fail, so use &error_abort.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20231123143813.42632-5-philmd@linaro.org>