Exeption message of 'live migration' is not appropriate

Fix bug 1194367

When do live-migration with RESTAPI body as:
{
    "os-migrateLive": {
        "host": "0443e9a1254044d8b99f35eace132080",
        "block_migration": false
    }
}

Customer would get the following exception:
{
    "badRequest": {
        "message": "host and block_migration must be specified.",
        "code": 400
    }
}

Update the exception to the following which is more accurate:
host, block_migration and disk_over_commit must be specified.

Change-Id: I87fbc8e9d287cd73084ae5e36dd7d6c0bcaec41c
Merge "Exeption message of 'live migration' is not appropriate"