Setup the interrupt controller

[andreeaflorescu]

Provide an interface for setting up the interrupt controller. In Firecracker we’re using the in-kernel interrupt controller on both platforms, but other products (Cloud Hypervisor, Crosvm) also support user space interrupt controllers (i.e. I/O APIC). We need to figure out if we need a unified interrupt interface on both platforms that supports in-kernel and user space interrupt controllers. As part of this task, we also need to figure out the best abstraction for creating the GIC on aarch64. The typical initialization flow tries to create a GICv3, and if that fails a GICv2 is created instead.