Reference: fate#315125:
Set NOCOW flag to newly created volume by default, to solve performance
issue on btrfs.

Signed-off-by: Chunyan Liu <cyliu@suse.com>

Index: virt-manager-1.2.0/virtinst/storage.py
===================================================================
--- virt-manager-1.2.0.orig/virtinst/storage.py
+++ virt-manager-1.2.0/virtinst/storage.py
@@ -705,6 +705,11 @@ class StorageVolume(_StorageObject):
     lazy_refcounts = XMLProperty("./target/features/lazy_refcounts",
         is_bool=True, default_cb=_lazy_refcounts_default_cb)
 
+    def _nocow_default_cb(self):
+        return True
+    nocow = XMLProperty("./target/nocow",
+        is_bool=True, default_cb=_nocow_default_cb)
+
 
     ######################
     # Public API helpers #