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-1.3.2/virtinst/storage.py
===================================================================
--- virt-manager-1.3.2.orig/virtinst/storage.py
+++ virt-manager-1.3.2/virtinst/storage.py
@@ -716,6 +716,12 @@ 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 self.conn.check_support(
+            self.conn.SUPPORT_CONN_NOCOW)
+    nocow = XMLProperty("./target/nocow",
+        is_bool=True, default_cb=_nocow_default_cb)
+
 
     ######################
     # Public API helpers #
Index: virt-manager-1.3.2/virtinst/support.py
===================================================================
--- virt-manager-1.3.2.orig/virtinst/support.py
+++ virt-manager-1.3.2/virtinst/support.py
@@ -314,6 +314,8 @@ SUPPORT_CONN_VCPU_PLACEMENT = _make(
 SUPPORT_CONN_MEM_STATS_PERIOD = _make(
     function="virDomain.setMemoryStatsPeriod",
     version="1.1.1", hv_version={"qemu": 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
 # limited to arbitrary new enough xen, since I know libxl can handle it