qmp-commands.hx: Document the missing options for migration capability commands

qmp-commands.hx: Document the missing options for migration capability commands

Add the missing descriptions for the options of migration capability commands,
and fix the example for query-migrate-capabilities command.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Message-Id: <1452829066-9764-7-git-send-email-zhang.zhanghailiang@huawei.com>
[Amit: Strip whitespace]
Signed-off-by: Amit Shah <amit.shah@redhat.com>