ARM: KVM: VGIC initialisation code

Add the init code for the hypervisor, the virtual machine, and
the virtual CPUs.

An interrupt handler is also wired to allow the VGIC maintenance
interrupts, used to deal with level triggered interrupts and LR
underflows.

A CPU hotplug notifier is registered to disable/enable the interrupt
as requested.

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