hw/arm/virt: Introduce virt_set_high_memmap() helper

hw/arm/virt: Introduce virt_set_high_memmap() helper

This introduces virt_set_high_memmap() helper. The logic of high
memory region address assignment is moved to the helper. The intention
is to make the subsequent optimization for high memory region address
assignment easier.

No functional change intended.

Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Tested-by: Zhenyu Zhang <zhenyzha@redhat.com>
Message-id: 20221029224307.138822-2-gshan@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>