xenapi: Retrieve VM uuid from xenstore.

Fall back to retrieving the uuid from xenstore if
/sys/hypervisor/uuid isn't accessible.

Change-Id: I409079068d3102ff86a71431b29c1ce2e6fe8857
Fixes: bug #1157211

nova/virt/xenapi/vm_utils.py

@@ -1918,11 +1918,25 @@ def vdi_attached_here(session, vdi_ref, read_only=False):
         LOG.debug(_('Destroying VBD for VDI %s done.'), vdi_ref)
 
 
-def get_this_vm_uuid():
+def _get_sys_hypervisor_uuid():
     with file('/sys/hypervisor/uuid') as f:
         return f.readline().strip()
 
 
+def get_this_vm_uuid():
+    try:
+        return _get_sys_hypervisor_uuid()
+    except IOError:
+        # Some guest kernels (without 5c13f8067745efc15f6ad0158b58d57c44104c25)
+        # cannot read from uuid after a reboot.  Fall back to trying xenstore.
+        # See https://bugs.launchpad.net/ubuntu/+source/xen-api/+bug/1081182
+        domid, _ = utils.execute('xenstore-read', 'domid', run_as_root=True)
+        vm_key, _ = utils.execute('xenstore-read',
+                                 '/local/domain/%s/vm' % domid.strip(),
+                                 run_as_root=True)
+        return vm_key.strip()[4:]
+
+
 def _get_this_vm_ref(session):
     return session.call_xenapi("VM.get_by_uuid", get_this_vm_uuid())