libvirt: Remove unnecessary arg

Spotted while tracing the flow of disk provisioning during boot. None of
the other callers of this method pass an argument, so there's no need to
pass it here. The parameter itself must be retained to support
management of existing images, however (e.g. snapshots).

Change-Id: I7829b062fc5abcb0f8584f5df2b5da2e3241e73a
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

nova/virt/libvirt/driver.py

@@ -1851,7 +1851,7 @@ class LibvirtDriver(driver.ComputeDriver):
         # the instance directory and disk.info on init, this condition will
         # need to be re-visited to make sure that backend doesn't re-create
         # the disk. Refer to bugs: 1666831 1728603 1769131
-        if self.image_backend.backend(CONF.libvirt.images_type).SUPPORTS_CLONE:
+        if self.image_backend.backend().SUPPORTS_CLONE:
             root_disk = self.image_backend.by_name(instance, 'disk')
             if root_disk.exists():
                 root_disk.remove_snap(libvirt_utils.RESIZE_SNAPSHOT_NAME)