Some of the migration flags really should never be changed by users
since the code makes fundamental assumptions about their use.
Now that live_migration_tunnelled is available, there shouldn't be any
remaining use cases for the live_migration_flag and block_migration_flag
options.
Deprecate these for removal in a future release.
Change-Id: I69c4bdf47a7b34b8dc8cfa58f3d0e0557b5ce072
Signed-off-by: Mark McLoughlin <markmc@redhat.com>