[2117813] Improve descriptive text of VM details while VM is off

Description of problem:
There are 3 descriptive texts for a stopped VM:
1. Guest agent is required
2. Not available
3. VirtualMachine is not running

Problems found:
1. "Guest agent is required" shows when VM is off, it should only show when VM is running without the guest agent installed.
2. Not clear when to use "Not available" and when to use "VirtualMachine is not running".
3. on a stopped VM overview tab, the hostname shows "Guest agent is required", but on details tab, the hostname shows "Not available".

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