if specifying migration_host to an Ipv6 address without brackets,
it was resolved to an incorrect address, such as:
    tcp:2001:0DB8::1428:4444,
but the correct address should be:
    tcp:[2001:0DB8::1428]:4444
so we should add brackets when parsing it.
Signed-off-by: NChen Fan <chen.fan.fnst@cn.fujitsu.com>