qmp-commands.hx: Fix the missing options for migration parameters commands

qmp-commands.hx: Fix the missing options for migration parameters commands

We didn't document x-cpu-throttle-initial/x-cpu-throttle-increment for
commands migrate-set-parameters and query-migrate-parameters.

Here we add the descriptions for these two options and fix the wrong example
for query-migrate-parameters qmp commands.
Besides, this will also fix the bug that we can't set x-cpu-throttle-initial
and x-cpu-throttle-increment through migrate-set-parameters qmp 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: <1452829066-9764-6-git-send-email-zhang.zhanghailiang@huawei.com>
[Amit: fix typo in 'auto-converge']
Signed-off-by: Amit Shah <amit.shah@redhat.com>