Error Detaching Volume on GlusterFS Primary Storage

##### ISSUE TYPE
* Bug Report

##### COMPONENT NAME

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Primary Storage (GlusterFS)
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##### CLOUDSTACK VERSION

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4.17.1.0
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##### CONFIGURATION

##### OS / ENVIRONMENT

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CentOS Linux 7 (Core)
qemu-img version 2.12.0
glusterfs 6.1

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##### SUMMARY

Can't detach volumes created on GlusterFS primary storage from VM. Tested on two different cloudstack installations. Everything works fine with other primary storage types. The volume can be detached and removed if the VM status is stopped but not when running.

##### STEPS TO REPRODUCE

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- Create GlusterFS Primary Storage with tag
- Create Disk Offering that uses just crated GlusterFS Primary Storage's tag.
- Create Volume using just created Disk Offering
- Attach Volume to VM
- Detach Volume from VM

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##### EXPECTED RESULTS

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- Success detaching Volume

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##### ACTUAL RESULTS

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- (test-detach) Failed to detach volume test-detach from VM test-vm-for-gluster ; com.cloud.exception.InternalErrorException: disk: /mnt/b440b605-a876-35f8-be84-2b5696650654/5605035d-e060-42a6-89a1-6f9c949463fc is not attached before

~~~

首先，在运行中的 VM 上使用 GlusterFS 作为主存储，并使用所列出的 CentOS、qemu-img 和 glusterfs 环境，复现 issue 中描述的 detach 流程。跟踪“is not attached before”错误的来源，并将其与其他主存储类型进行比较。当 VM 运行时 volume 能够成功 detach，且报告的场景已由回归测试覆盖时，即表示完成。

由索引模型根据 Issue 内容生成。