lxc: Fix error handlings in lxcContainerRenameAndEnableInterfaces

The function is expected to return negative value on failure,
however, it returns positive value when either setInterfaceName
or vethInterfaceUpOrDown fails. Because the function returns
the return value of either as is, however, the two functions
may return positive value on failure.

The patch fixes the defects and add error messages.

src/lxc/lxc_container.c

@@ -255,13 +255,20 @@ static int lxcContainerRenameAndEnableInterfaces(unsigned int nveths,
 
         DEBUG("Renaming %s to %s", veths[i], newname);
         rc = setInterfaceName(veths[i], newname);
-        if (0 != rc)
+        if (0 != rc) {
+            VIR_ERROR(_("Failed to rename %s to %s (%d)"),
+                      veths[i], newname, rc);
+            rc = -1;
             goto error_out;
+        }
 
         DEBUG("Enabling %s", newname);
-        rc =  vethInterfaceUpOrDown(newname, 1);
-        if (0 != rc)
+        rc = vethInterfaceUpOrDown(newname, 1);
+        if (0 != rc) {
+            VIR_ERROR(_("Failed to enable %s (%d)"), newname, rc);
+            rc = -1;
             goto error_out;
+        }
         VIR_FREE(newname);
     }