ARM: KVM: VGIC interrupt injection

Plug the interrupt injection code. Interrupts can now be generated
from user space.

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>