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-1.4.0/virtinst/storage.py
===================================================================
--- virt-manager-1.4.0.orig/virtinst/storage.py
+++ virt-manager-1.4.0/virtinst/storage.py
@@ -707,6 +707,12 @@ class StorageVolume(_StorageObject):
         return self.TYPE_FILE
     file_type = property(_get_vol_type)
 
+    def _nocow_default_cb(self):
+        return self.conn.check_support(
+            self.conn.SUPPORT_CONN_NOCOW)
+    nocow = XMLProperty("./target/nocow",
+        is_bool=True, default_cb=_nocow_default_cb)
+
 
     ##################
     # XML properties #
Index: virt-manager-1.4.0/virtinst/support.py
===================================================================
--- virt-manager-1.4.0.orig/virtinst/support.py
+++ virt-manager-1.4.0/virtinst/support.py
@@ -316,6 +316,8 @@ SUPPORT_CONN_SPICE_GL = _make(version="1
 SUPPORT_CONN_VIDEO_VIRTIO_ACCEL3D = _make(version="1.3.0",
     hv_version={"qemu": "2.7.0", "test": 0})
 SUPPORT_CONN_GRAPHICS_LISTEN_NONE = _make(version="2.0.0")
+SUPPORT_CONN_NOCOW = _make(
+    version="1.2.18", hv_version={"qemu": "2.2.0", "test": 0})
 
 
 # This is for disk <driver name=qemu>. xen supports this, but it's