KVM: arm/arm64: Remove struct vgic_irq pending field

One of the goals behind the VGIC redesign was to get rid of cached or
intermediate state in the data structures, but we decided to allow
ourselves to precompute the pending value of an IRQ based on the line
level and pending latch state.  However, this has now become difficult
to base proper GICv3 save/restore on, because there is a potential to
modify the pending state without knowing if an interrupt is edge or
level configured.

See the following post and related message for more background:
https://lists.cs.columbia.edu/pipermail/kvmarm/2017-January/023195.html

This commit gets rid of the precomputed pending field in favor of a
function that calculates the value when needed, irq_is_pending().

The soft_pending field is renamed to pending_latch to represent that
this latch is the equivalent hardware latch which gets manipulated by
the input signal for edge-triggered interrupts and when writing to the
SPENDR/CPENDR registers.

After this commit save/restore code should be able to simply restore the
pending_latch state, line_level state, and config state in any order and
get the desired result.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>