KVM: arm/arm64: vgic-new: Add GICv3 world switch backend

As the GICv3 virtual interface registers differ from their GICv2
siblings, we need different handlers for processing maintenance
interrupts and reading/writing to the LRs.
Implement the respective handler functions and connect them to
existing code to be called if the host is using a GICv3.

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