Correct extend and append in check_windows_vm_hvinfo (RedHatQE#2634)

##### Short description:
Correct extend and append in check_windows_vm_hvinfo 

##### More details:
Extand should be used on lists while append should be used on strings,
this PR aim to fix those in check_windows_vm_hvinfo

##### Which issue(s) this PR fixes:

RedHatQE#2624 (comment)


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Fixed VM recommendation failure reporting so all individual failures
are listed clearly rather than grouped or nested.
* Corrected Hyper‑V support validation so a single clear error is
recorded when support is missing, avoiding fragmented or character-level
error entries.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: rkishner <rkishner@redhat.com>

Author: RoniKishner

tests/virt/cluster/common_templates/utils.py

@@ -531,7 +531,7 @@ def _check_hyperv_recommendations():
         ]
 
         if failed_vm_recommendations:
-            failed_recommendations.append(failed_vm_recommendations)
+            failed_recommendations.extend(failed_vm_recommendations)
 
         spinlocks = vm_recommendations_dict["SpinlockRetries"]
         if int(spinlocks) != 8191:
@@ -575,7 +575,7 @@ def _check_hyperv_features():
     failed_windows_hyperv_list.extend(_check_hyperv_features())
 
     if not hvinfo_dict["HyperVsupport"]:
-        failed_windows_hyperv_list.extend("HyperVsupport")
+        failed_windows_hyperv_list.append("HyperVsupport")
 
     assert not failed_windows_hyperv_list, (
         f"The following hyperV flags are not set correctly in the guest: {failed_windows_hyperv_list}\n"