arm/arm64: KVM: vgic: Factor out level irq processing on guest exit

Currently vgic_process_maintenance() processes dealing with a completed
level-triggered interrupt directly, but we are soon going to reuse this
logic for level-triggered mapped interrupts with the HW bit set, so
move this logic into a separate static function.

Probably the most scary part of this commit is convincing yourself that
the current flow is safe compared to the old one.  In the following I
try to list the changes and why they are harmless:

  Move vgic_irq_clear_queued after kvm_notify_acked_irq:
    Harmless because the only potential effect of clearing the queued
    flag wrt.  kvm_set_irq is that vgic_update_irq_pending does not set
    the pending bit on the emulated CPU interface or in the
    pending_on_cpu bitmask if the function is called with level=1.
    However, the point of kvm_notify_acked_irq is to call kvm_set_irq
    with level=0, and we set the queued flag again in
    __kvm_vgic_sync_hwstate later on if the level is stil high.

  Move vgic_set_lr before kvm_notify_acked_irq:
    Also, harmless because the LR are cpu-local operations and
    kvm_notify_acked only affects the dist

  Move vgic_dist_irq_clear_soft_pend after kvm_notify_acked_irq:
    Also harmless, because now we check the level state in the
    clear_soft_pend function and lower the pending bits if the level is
    low.

Reviewed-by: Eric Auger <eric.auger@linaro.org>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>