Add virFileTouch for creating empty files

Add a virFileTouch API which ensures that a file will always
exist, even if zero length

* src/util/virfile.c, src/util/virfile.h,
  src/libvirt_private.syms: Introduce virFileTouch

src/libvirt_private.syms

@@ -1174,6 +1174,7 @@ virFileDirectFdNew;
 virFileFclose;
 virFileFdopen;
 virFileRewrite;
+virFileTouch;
 
 
 # virkeycode.h

src/util/virfile.c

@@ -390,3 +390,24 @@ cleanup:
     }
     return ret;
 }
+
+
+int virFileTouch(const char *path, mode_t mode)
+{
+    int fd = -1;
+
+    if ((fd = open(path, O_WRONLY | O_CREAT, mode)) < 0) {
+        virReportSystemError(errno, _("cannot create file '%s'"),
+                             path);
+        return -1;
+    }
+
+    if (VIR_CLOSE(fd) < 0) {
+        virReportSystemError(errno, _("cannot save file '%s'"),
+                             path);
+        VIR_FORCE_CLOSE(fd);
+        return -1;
+    }
+
+    return 0;
+}

src/util/virfile.h

@@ -74,4 +74,6 @@ int virFileRewrite(const char *path,
                    virFileRewriteFunc rewrite,
                    void *opaque);
 
+int virFileTouch(const char *path, mode_t mode);
+
 #endif /* __VIR_FILES_H */