KVM: arm/arm64: vgic: Convert struct vgic_lr to use bitfields
As we're about to cram more information in the vgic_lr structure
(HW interrupt number and additional state information), we switch
to a layout similar to the HW's:
- use bitfields to save space (we don't need more than 10 bits
to represent the irq numbers)
- source CPU and HW interrupt can share the same field, as
a SGI doesn't have a physical line.