hw/arm/virt-acpi-build: Factor out create_its_idmaps

hw/arm/virt-acpi-build: Factor out create_its_idmaps

Factor out a new function, create_its_idmaps(), from the current
build_iort code. Add proper comments to it clarifying how the ID ranges
that go directly to the ITS Group node are computed based on the ones
that are directed to the SMMU node.

Suggested-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>
Message-id: 20250628195722.977078-6-gustavo.romero@linaro.org
[PMM: drop hardcoded tabs]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>