virfile: Add error for root squash change mode failure

This will only be seen when debugging, but in order to help determine
whether a virFileOpenForceOwnerMode failed during an NFS root-squash
volume/file creation, add an error message from the child.

src/util/virfile.c

@@ -2113,8 +2113,13 @@ virFileOpenForked(const char *path, int openflags, mode_t mode,
 
         /* File is successfully open. Set permissions if requested. */
         ret = virFileOpenForceOwnerMode(path, fd, mode, uid, gid, flags);
-        if (ret < 0)
+        if (ret < 0) {
+            ret = -errno;
+            virReportSystemError(errno,
+                                 _("child process failed to force owner mode file '%s'"),
+                                 path);
             goto childerror;
+        }
 
         do {
             ret = sendfd(pair[1], fd);