Rather than having the virt machine model create an a15mpcore_priv
device regardless of the actual CPU type in order to instantiate the GIC,
move to having the machine model create the GIC directly. This
corresponds to a system which uses a standalone GIC (eg the GIC-400)
rather than the one built in to the CPU core.

The primary motivation for this is to support the Cortex-A57,
which for a KVM configuration will use a GICv2, which is not
built into the CPU.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Message-id: 1398362083-17737-2-git-send-email-peter.maydell@linaro.org