Add support for "-cpu max" for ARM guests. This CPU type behaves
like "-cpu host" when KVM is enabled, and like a system CPU with
the maximum possible feature set otherwise. (Note that this means
it won't be migratable across versions, as we will likely add
features to it in future.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20180308130626.12393-4-peter.maydell@linaro.org