hw/arm/virt: Introduce virt_get_high_memmap_enabled() helper

hw/arm/virt: Introduce virt_get_high_memmap_enabled() helper

This introduces virt_get_high_memmap_enabled() helper, which returns
the pointer to vms->highmem_{redists, ecam, mmio}. The pointer will
be used in the subsequent patches.

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: 20221029224307.138822-5-gshan@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>