Fix error message in pre_live_migration.

We are using locals() to reference a variable which doesn't exist
in this error message. Also rename the instance variable while I
am here to be more consistent with other uses.

Resolves bug 1164072.

Change-Id: I96f5e2a81ac2e97181c3e87df83e19b381f4bcd2
Merge "Fix error message in pre_live_migration."