KVM: arm/arm64: vgic-new: Add TARGET registers handlers

The target register handlers are v2 emulation specific, so their
implementation lives entirely in vgic-mmio-v2.c.
We copy the old VGIC behaviour of assigning an IRQ to the first VCPU
set in the target mask instead of making it possibly pending on
multiple VCPUs.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>