hw/arm/virt-acpi-build: remove unnecessary variable mcfg_start

mcfg_start points to the start of MCFG table and is used in
build_header. While this information could be derived from mcfg.

This patch removes the unnecessary variable mcfg_start.
Signed-off-by: NWei Yang <richardw.yang@linux.intel.com>
Reviewed-by: NIgor Mammedov <imammedo@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190419003053.8260-3-richardw.yang@linux.intel.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/arm/virt-acpi-build.c

@@ -552,7 +552,6 @@ build_mcfg(GArray *table_data, BIOSLinker *linker, VirtMachineState *vms)
     const MemMapEntry *memmap = vms->memmap;
     int ecam_id = VIRT_ECAM_ID(vms->highmem_ecam);
     int len = sizeof(*mcfg) + sizeof(mcfg->allocation[0]);
-    int mcfg_start = table_data->len;
 
     mcfg = acpi_data_push(table_data, len);
     mcfg->allocation[0].address = cpu_to_le64(memmap[ecam_id].base);
@@ -563,8 +562,7 @@ build_mcfg(GArray *table_data, BIOSLinker *linker, VirtMachineState *vms)
     mcfg->allocation[0].end_bus_number =
         PCIE_MMCFG_BUS(memmap[ecam_id].size - 1);
 
-    build_header(linker, table_data, (void *)(table_data->data + mcfg_start),
-                 "MCFG", table_data->len - mcfg_start, 1, NULL, NULL);
+    build_header(linker, table_data, (void *)mcfg, "MCFG", len, 1, NULL, NULL);
 }
 
 /* GTDT */