libvirt: ignore deleted domain while get block dev

The get_all_block_devices could raise libvirtError if domain was deleted
while listing instances. This will cause LibvirtISCSIVolumeDriver to fail
disconnect_volume. This patch ignores the domain that could not be found
to avoid this issue.

Fixes bug 1060836

Change-Id: I9dc994d8ca7fc084830adb4f6f9d3ca10e0fe577

nova/virt/libvirt/driver.py

@@ -1899,8 +1899,8 @@ def get_all_block_devices(self):
         """
         devices = []
         for dom_id in self.list_instance_ids():
-            domain = self._conn.lookupByID(dom_id)
             try:
+                domain = self._conn.lookupByID(dom_id)
                 doc = etree.fromstring(domain.XMLDesc(0))
             except Exception:
                 continue