more realistic disk size for Ceph primary storage

### The required feature described as a wish

On Cloudstack 4.22.1.0 I created a RBD primary storage on a RADOS pool named "cloudstack-ssd".
Ceph reports that the pool used percentage is 5.17 and the maximum available capacity is 2.6 TiB.
```
# ceph df
--- RAW STORAGE ---
CLASS SIZE AVAIL USED RAW USED %RAW USED
hdd 33 TiB 31 TiB 1.3 TiB 1.3 TiB 3.89
ssd 8.7 TiB 8.3 TiB 447 GiB 447 GiB 5.00
TOTAL 41 TiB 40 TiB 1.7 TiB 1.7 TiB 4.12

--- POOLS ---
POOL ID PGS STORED OBJECTS USED %USED MAX AVAIL
cloudstack-ssd 4 32 169 GiB 48.15k 438 GiB 5.17 2.6 TiB
cloudstack-hdd 5 32 427 GiB 111.94k 1.2 TiB 3.96 9.7 TiB
```

But Cloudstack reports that "cloudstack-ssd" primary storage size is 42476.68 GB (about 41 TiB), which is Ceph total raw capacity.

I think that the value reported by Cloudstack is misleading.
It's not possible to compute the exact pool size because the raw space is contended between different pools.
But a better option could be that for a RBD storage Cloudstack displays the pool maximum available size and the used percentage (2.6 TiB and 5.17% in my configuration) instead of the total raw Ceph size (41 TiB in my configuration).

首先定位 CloudStack 的 RBD 主要儲存容量報告，以及它如何取得 Ceph pool 的值。將目前的原始總容量與此處所述 pool 的 MAX AVAIL 和 USED 值進行比較；當主要儲存顯示反映特定 pool 的容量和使用量，且不會錯誤表示共用的原始容量時，工作即完成。

由索引模型根據 Issue 內容生成。