xen/arm: io: Distinguish unhandled IO from aborted one

xen/arm: io: Distinguish unhandled IO from aborted one

Currently, Xen is considering that an IO could either be handled or
unhandled. When unhandled, the stage-2 abort function will try another
way to resolve the abort.

However, the MMIO emulation may return unhandled when the address
belongs to an emulated range but was not correct. In that case, Xen
should avoid to try another way and directly inject a guest data abort.

Introduce a tri-state return to distinguish the following state:
    * IO_ABORT: The IO was handled but resulted in an abort
    * IO_HANDLED: The IO was handled
    * IO_UNHANDLED: The IO was unhandled

For now, it is considered that an IO belonging to an emulated range
could either be handled or inject an abort. This could be revisit in the
future if overlapped region exist (or we want to try another way to
resolve the abort).

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