conf: Fix prototype/definition for virStoragePoolObj get functions

Modify virStoragePoolObjGetAutostartLink and
virStoragePoolObjGetConfigFile to return "const char *"
since that's how both are used and to ensure no one
tries to VIR_FREE the result.

src/conf/virstorageobj.c

@@ -89,7 +89,7 @@ virStoragePoolObjDefUseNewDef(virStoragePoolObjPtr obj)
 }
 
 
-char *
+const char *
 virStoragePoolObjGetConfigFile(virStoragePoolObjPtr obj)
 {
     return obj->configFile;
@@ -105,7 +105,7 @@ virStoragePoolObjSetConfigFile(virStoragePoolObjPtr obj,
 }
 
 
-char *
+const char *
 virStoragePoolObjGetAutostartLink(virStoragePoolObjPtr obj)
 {
     return obj->autostartLink;

src/conf/virstorageobj.h

@@ -86,14 +86,14 @@ virStoragePoolObjGetNewDef(virStoragePoolObjPtr obj);
 void
 virStoragePoolObjDefUseNewDef(virStoragePoolObjPtr obj);
 
-char *
+const char *
 virStoragePoolObjGetConfigFile(virStoragePoolObjPtr obj);
 
 void
 virStoragePoolObjSetConfigFile(virStoragePoolObjPtr obj,
                                char *configFile);
 
-char *
+const char *
 virStoragePoolObjGetAutostartLink(virStoragePoolObjPtr obj);
 
 bool