This only implements the external delivery method via the GIC.

Acked-by: Greg Bellows <greg.bellows@linaro.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 1411718914-6608-12-git-send-email-edgar.iglesias@gmail.com
[PMM: adjusted following cpu-exec refactoring]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>