acpi: vmgenid_build_acpi: use acpi_table_begin()/acpi_table_end() instead of build_header()

it replaces error-prone pointer arithmetic for build_header() API,
with 2 calls to start and finish table creation,
which hides offsets magic from API user.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Message-Id: <20210924122802.1455362-13-imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/acpi/vmgenid.c

@@ -29,6 +29,8 @@ void vmgenid_build_acpi(VmGenIdState *vms, GArray *table_data, GArray *guid,
     Aml *ssdt, *dev, *scope, *method, *addr, *if_ctx;
     uint32_t vgia_offset;
     QemuUUID guid_le;
+    AcpiTable table = { .sig = "SSDT", .rev = 1,
+                        .oem_id = oem_id, .oem_table_id = "VMGENID" };
 
     /* Fill in the GUID values.  These need to be converted to little-endian
      * first, since that's what the guest expects
@@ -42,12 +44,10 @@ void vmgenid_build_acpi(VmGenIdState *vms, GArray *table_data, GArray *guid,
     g_array_insert_vals(guid, VMGENID_GUID_OFFSET, guid_le.data,
                         ARRAY_SIZE(guid_le.data));
 
-    /* Put this in a separate SSDT table */
+    /* Put VMGNEID into a separate SSDT table */
+    acpi_table_begin(&table, table_data);
     ssdt = init_aml_allocator();
 
-    /* Reserve space for header */
-    acpi_data_push(ssdt->buf, sizeof(AcpiTableHeader));
-
     /* Storage for the GUID address */
     vgia_offset = table_data->len +
         build_append_named_dword(ssdt->buf, "VGIA");
@@ -116,9 +116,8 @@ void vmgenid_build_acpi(VmGenIdState *vms, GArray *table_data, GArray *guid,
         ACPI_BUILD_TABLE_FILE, vgia_offset, sizeof(uint32_t),
         VMGENID_GUID_FW_CFG_FILE, 0);
 
-    build_header(linker, table_data,
-        (void *)(table_data->data + table_data->len - ssdt->buf->len),
-        "SSDT", ssdt->buf->len, 1, oem_id, "VMGENID");
+    /* must be called after above command to ensure correct table checksum */
+    acpi_table_end(linker, &table);
     free_aml_allocator();
 }