target-arm: Allow secondary KVM CPUs to be booted via PSCI

New ARM boards are generally expected to boot their secondary CPUs
via the PSCI interface, rather than ad-hoc "loop around in holding
pen code" as hw/arm/boot.c implements. In particular this is
necessary for mach-virt kernels. For KVM we achieve this by creating
the VCPUs with a feature flag marking them as starting in PSCI
powered-down state; the guest kernel will then make a PSCI call
(implemented in the host kernel) to start the secondaries at
an address of its choosing once it has got the primary CPU up.

Implement this setting of the feature flag, controlled by a
qdev property for ARMCPU, which board code can set if it is a
PSCI system.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Message-id: 1385140638-10444-7-git-send-email-peter.maydell@linaro.org
