hw/arm/virt: Minor code reshuffling in create_acpi_ged

hw/arm/virt: Minor code reshuffling in create_acpi_ged

Use a local SysBusDevice handle. Also use the newly introduced
sysbus_mmio_map_name which brings better readability about the region
being mapped. GED device has regions which exist depending on some
external properties and it becomes difficult to guess the index of
a region. Better refer to a region by its name.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20250714080639.2525563-32-eric.auger@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>