Allow limiting IO operation size (KVM)

##### ISSUE TYPE

* Improvement Request

##### COMPONENT NAME

~~~
KVM hypervisor
~~~

##### CLOUDSTACK VERSION

~~~
4.18 or latest
~~~

##### SUMMARY

The [qemu documentation](https://github.com/qemu/qemu/blob/master/docs/throttle.txt#L142) explains why this is important:

> When applying IOPS limits all I/O operations are treated equally
regardless of their size. This means that the user can take advantage
of this in order to circumvent the limits and submit one huge I/O
request instead of several smaller ones.
>
> QEMU provides a setting called throttling.iops-size to prevent this
from happening. This setting specifies the size (in bytes) of an I/O
request for accounting purposes. Larger requests will be counted
proportionally to this size.

In libvirt this option is called `size_iops_sec` inside the `` section ([ref](https://libvirt.org/formatdomain.html)).

Other big clouds such as AWS also use this mechanism to fairly allocate IOPS.

##### STEPS TO REPRODUCE

Checking the CloudStack code shows that this option is currently not supported:
https://github.com/apache/cloudstack/blob/1411da1a22bc6aa26634f3038475e3d5fbbcd6bb/plugins/hypervisors/kvm/src/main/java/com/cloud/hypervisor/kvm/resource/LibvirtVMDef.java#L1173-L1207

从 plugins/hypervisors/kvm/src/main/java/com/cloud/hypervisor/kvm/resource/LibvirtVMDef.java 的 1173-1207 行附近开始，issue 中指出 KVM IO 选项就在此处定义。阅读链接的 QEMU 限流文档和 libvirt 的 size_iops_sec 参考，然后使该选项可用于 CloudStack 的 KVM 配置，并验证生成的设置使用请求的 IO 大小进行计量。

由索引模型根据 Issue 内容生成。