kvm: do not add disk cache mode from an empty string

[DaanHoogland]

Description

This PR fixes a bug discussed in #12226 (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

Feature/Enhancement Scale

• Major
• Minor

Bug Severity

• BLOCKER
• Critical
• Major
• Minor
• Trivial

Screenshots (if appropriate):

How Has This Been Tested?

How did you try to break this feature and the system with this change?

[weizhouapache]

code lgtm

I will test it

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 16.23%. Comparing base (aba3285) to head (a573bed).

Files with missing lines	Patch %	Lines
...ypervisor/kvm/resource/LibvirtDomainXMLParser.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               4.20   #12248      +/-   ##
============================================
- Coverage     16.23%   16.23%   -0.01%     
+ Complexity    13382    13380       -2     
============================================
  Files          5657     5657              
  Lines        498997   498998       +1     
  Branches      60566    60567       +1     
============================================
- Hits          81032    81028       -4     
- Misses       408929   408934       +5     
  Partials       9036     9036              

Flag	Coverage Δ
uitests	4.03% <ø> (ø)
unittests	17.09% <0.00%> (-0.01%)	⬇️

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[weizhouapache]

@blueorangutan package

[sureshanaparti]

clgtm

[weizhouapache]

@blueorangutan test

[weizhouapache]

@blueorangutan test

[vishesh92]

clgtm

[DaanHoogland]

@blueorangutan package

[blueorangutan]

@DaanHoogland a [SL] Jenkins job has been kicked to build packages. It will be bundled with KVM, XenServer and VMware SystemVM templates. I'll keep you posted as I make progress.

[blueorangutan]

Packaging result [SF]: ✔️ el8 ✔️ el9 ✔️ el10 ✔️ debian ✔️ suse15. SL-JID 16378

[DaanHoogland]

@blueorangutan test

[blueorangutan]

@DaanHoogland a [SL] Trillian-Jenkins test job (ol8 mgmt + kvm-ol8) has been kicked to run smoke tests