Improvement: Filesystem permission check

##### ISSUE TYPE
* Improvement Request

##### COMPONENT NAME

~~~
cloudstack-sysvmadm
~~~

##### CLOUDSTACK VERSION
~~~
N/A
~~~

##### CONFIGURATION

N/A

##### OS / ENVIRONMENT
Cloudstack running on linux systems

##### SUMMARY
Check filesystem permission before upgrading systemvms

##### STEPS TO REPRODUCE

Currently, when issuing the cloudstack-sysvmadm script for upgrading the systemvms (or doing it manually), the process starts without any prechecks.
When there is somehow an issue with filesystem permissions, the new systemvm.iso cannot be copied to the new location. It would be a nice enhancement, when cloudstack had a health check or at least the cloudstack-sysvmadm script a permission check.

A newly introduced health check could continuously check some relevant cloudstack metrics, including the filesystem permissions and show a simple go/nogo somewhere in the admin ui. Or the simple way: a script or the modified sysvmadm script which checks first all storage paths for the correct rights.

~~~

~~~

##### EXPECTED RESULTS

~~~

~~~

##### ACTUAL RESULTS

~~~

~~~

Start by locating the cloudstack-sysvmadm entry point and how it discovers storage paths before upgrading system VMs. Review the existing upgrade flow and determine the expected permission checks and failure behavior. Done means the chosen script or health check detects unusable permissions before the systemvm.iso copy or upgrade begins.

Written by the indexing model from the issue text.