Fix misleading error message when trying to restore and attach volumes from backups

[bernardodemarco]

Description

This PR fixes/improves an error message that is thrown when it is attempted to restore and attach volumes from backups to a target VM that is attached to a backup offering. The message mentions that the target VM has backups, but that is not necessarily true. If they are simply attached to a backup offering, the error message will already be emitted.

I have run into this situation when testing KBOSS (see #12758 (comment)).

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)
• Build/CI
• Test (unit or integration test code)

Feature/Enhancement Scale or Bug Severity

Bug Severity

• BLOCKER
• Critical
• Major
• Minor
• Trivial

Screenshots (if appropriate):

How Has This Been Tested?

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 16.26%. Comparing base (4244c2c) to head (c3db401).

Files with missing lines	Patch %	Lines
...rg/apache/cloudstack/backup/BackupManagerImpl.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               4.20   #13450   +/-   ##
=========================================
  Coverage     16.25%   16.26%           
- Complexity    13432    13434    +2     
=========================================
  Files          5667     5667           
  Lines        500670   500670           
  Branches      60801    60801           
=========================================
+ Hits          81397    81423   +26     
+ Misses       410165   410140   -25     
+ Partials       9108     9107    -1     

Flag	Coverage Δ
uitests	4.14% <ø> (ø)
unittests	17.11% <0.00%> (+<0.01%)	⬆️

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