Fix migrating instance to the same host.

In current code logic, Migration to the same host is allowed which is
meaningless.

In the patch, a Conf flag 'allow_migrate_to_same_host' is added and set to
false as default. When 'allow_migrate_to_same_host' is false, the current
instance host will be added into filter property 'ignore_hosts' to avoid
migrating instance to the same host.

Fixes: bug #1160268

Change-Id: I5ac0391cf468310a89fb6d8a8f5120c5654cb4ff
Merge "Fix migrating instance to the same host."