ARM: new VGIC: Add ACTIVE registers handlers

ARM: new VGIC: Add ACTIVE registers handlers

The active register handlers are shared between the v2 and v3 emulation,
so their implementation goes into vgic-mmio.c, to be easily referenced
from the v3 emulation as well later.
Since activation/deactivation of an interrupt may happen entirely in the
guest without it ever exiting, we need some extra logic to properly track
the active state.
For clearing the active state, we would basically have to halt the guest
to make sure this is properly propagated into the respective VCPUs.
This is not yet implemented in Xen.
Fortunately this feature is mostly used to reset a just in initialised
GIC, so chances are we are tasked to clear bits that are already zero.
Add a simple check to avoid pointless warnings in this case.

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>