cpu: Use virCPUData.arch in cpuDecode

virCPUDef.arch is not required to be filled in for guest CPU
definitions. It doesn't make sense to artificially mandate it to be set
when cpuDecode is called especially when virCPUData.arch passed to
cpuDecode already contains the architecture.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>

src/cpu/cpu.c

@@ -240,7 +240,7 @@ cpuDecode(virCPUDefPtr cpu,
         return -1;
     }
 
-    if ((driver = cpuGetSubDriver(cpu->arch)) == NULL)
+    if ((driver = cpuGetSubDriver(data->arch)) == NULL)
         return -1;
 
     if (driver->decode == NULL) {