Add boolean convertors for migrate_live API

migrate_live API contains "block_migration" and "disk_over_commit" as
the parameters. They should be boolean.
There are the other boolean parameters such as "on_shared_storage" of
evacuate API, they can be passed with either boolean or string(e.g. "True").
For API consistency, this patch adds boolean convertors to migrate_live
API.

Change-Id: Ic3040811977c834bfd4b0b4527795382e11227ed
Closes-Bug: #1257661
Merge "Add boolean convertors for migrate_live API"