ARM: KVM: VGIC distributor handling

Add the GIC distributor emulation code. A number of the GIC features
are simply ignored as they are not required to boot a Linux guest.

Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>