conf: Add return value check to virResctrlAllocForeachCache

Add return value check to virResctrlAllocForeachCache in
virDomainCachetuneDefFormat. The virResctrlAllocForeachCache does have
return value, so need check return value to make sure function executed
without error.
Signed-off-by: NBing Niu <bing.niu@intel.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/conf/domain_conf.c

@@ -27294,10 +27294,10 @@ virDomainCachetuneDefFormat(virBufferPtr buf,
     int ret = -1;
 
     virBufferSetChildIndent(&childrenBuf, buf);
-    virResctrlAllocForeachCache(resctrl->alloc,
-                                virDomainCachetuneDefFormatHelper,
-                                &childrenBuf);
-
+    if (virResctrlAllocForeachCache(resctrl->alloc,
+                                    virDomainCachetuneDefFormatHelper,
+                                    &childrenBuf) < 0)
+        goto cleanup;
 
     if (virBufferCheckError(&childrenBuf) < 0)
         goto cleanup;