Use default qemu-img cluster size in libvirt connection driver

The recommended cluster size [1] for qcow2 is 64k, which is the default
when invoking 'qemu-img create ...'.  Remove the cluster_size option
from qemu-img invocation.

[1] http://www.mail-archive.com/openstack@lists.launchpad.net/msg11371.html

V2: Fix test.

Note: This patch is needed in the stable/essex branch too, otherwise it
is not possible to start Xen instances that use qcow2 images.  The old
qemu 0.10 in Xen cannot cope with cluster size > 64k.

Change-Id: Iafc290720a820cb5f57ded10a2f584fdc5cd5b49
(cherry picked from commit de5cb27e64)

Authors

@@ -84,6 +84,7 @@ Jason Koelker <jason@koelker.net>
 Jay Pipes <jaypipes@gmail.com>
 JC Martin <jcmartin@ebaysf.com>
 Jesse Andrews <anotherjesse@gmail.com>
+Jim Fehlig <jfehlig@suse.com>
 Jimmy Bergman <jimmy@sigint.se>
 Joe Gordon <jogo@cloudscaling.com>
 Joe Heck <heckj@mac.com>

nova/tests/test_libvirt.py

@@ -2135,7 +2135,7 @@ class LibvirtUtilsTestCase(test.TestCase):
     def test_create_cow_image(self):
         self.mox.StubOutWithMock(utils, 'execute')
         utils.execute('qemu-img', 'create', '-f', 'qcow2',
-                      '-o', 'cluster_size=2M,backing_file=/some/path',
+                      '-o', 'backing_file=/some/path',
                       '/the/new/cow')
         # Start test
         self.mox.ReplayAll()

nova/virt/libvirt/utils.py

@@ -69,7 +69,7 @@ def create_cow_image(backing_file, path):
     :param path: Desired location of the COW image
     """
     execute('qemu-img', 'create', '-f', 'qcow2', '-o',
-             'cluster_size=2M,backing_file=%s' % backing_file, path)
+             'backing_file=%s' % backing_file, path)
 
 
 def get_disk_size(path):