hw/intc/gicv3: Add data fields for virtualization support

As the first step in adding support for the virtualization
extensions to the GICv3 emulation:
 * add the necessary data fields to the state structures
 * add the fields to the migration state, as a subsection
   which is only present if virtualization is enabled

The use of a subsection means we retain migration
compatibility as EL2 is not enabled on any CPUs currently.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 1483977924-14522-8-git-send-email-peter.maydell@linaro.org