arm/arm64: KVM: vgic: Clarify and correct vgic documentation

 * How the whole thing works (courtesy of Christoffer Dall):

 *

 * - At any time, the dist->irq_pending_on_cpu is the oracle that knows if

 *   something is pending

 * - VGIC pending interrupts are stored on the vgic.irq_pending vgic

 *   bitmap (this bitmap is updated by both user land ioctls and guest

 *   mmio ops, and other in-kernel peripherals such as the

 *   arch. timers) and indicate the 'wire' state.

 *   something is pending on the CPU interface.

 * - Interrupts that are pending on the distributor are stored on the

 *   vgic.irq_pending vgic bitmap (this bitmap is updated by both user land

 *   ioctls and guest mmio ops, and other in-kernel peripherals such as the

 *   arch. timers).

 * - Every time the bitmap changes, the irq_pending_on_cpu oracle is

 *   recalculated

 * - To calculate the oracle, we need info for each cpu from

 *   compute_pending_for_cpu, which considers:

 *   - PPI: dist->irq_pending & dist->irq_enable

 *   - SPI: dist->irq_pending & dist->irq_enable & dist->irq_spi_target

 *   - irq_spi_target is a 'formatted' version of the GICD_ICFGR

 *   - irq_spi_target is a 'formatted' version of the GICD_ITARGETSRn

 *     registers, stored on each vcpu. We only keep one bit of

 *     information per interrupt, making sure that only one vcpu can

 *     accept the interrupt.

 * - If any of the above state changes, we must recalculate the oracle.

 * - The same is true when injecting an interrupt, except that we only

 *   consider a single interrupt at a time. The irq_spi_cpu array

 *   contains the target CPU for each SPI.