<fix>[kvm]: prevent metadata deletion when DVD returns empty

When hypervisor metadata collection from DVD returns an empty
list, the existing metadata was deleted before the empty check,
causing all hosts to lose their hypervisor version metadata and
matchState to become Unknown.

1. Why is this change necessary?
saveHostOsCategoryList first deleted all existing metadata for
the management node, then checked if the input was empty. When
DVD collection returned empty, this wiped all metadata, causing
matchTargetVersion to be null and matchState to become Unknown.

2. How does it address the problem?
Move the empty-list check before the delete operation so that
an empty input preserves existing metadata. A warning is logged
to indicate the skip.

3. Are there any side effects?
None. Non-empty input behavior is unchanged.

# Summary of changes (by module):
- kvm: move empty check before metadata delete in
  KvmHypervisorInfoManagerImpl.saveHostOsCategoryList()

Related: ZSTAC-83682
Change-Id: I81f9baacac7fce9af2363a0ce5c960532d383890

Author: PandaWuu

plugin/kvm/src/main/java/org/zstack/kvm/hypervisor/KvmHypervisorInfoManagerImpl.java

@@ -275,13 +275,15 @@ private boolean saveMetadataList(List<HypervisorMetadataDefinition> definitions)
 
     @Transactional
     protected boolean saveHostOsCategoryList(List<HostOsCategoryVO> categoryVOS) {
+        if (CollectionUtils.isEmpty(categoryVOS)) {
+            logger.warn("no hypervisor metadata collected from DVD, skip refresh to preserve existing metadata");
+            return false;
+        }
+
         // refresh all metadata with current management node
         SQL.New(KvmHostHypervisorMetadataVO.class)
                 .eq(KvmHostHypervisorMetadataVO_.managementNodeUuid, Platform.getManagementServerId())
                 .delete();
-        if (CollectionUtils.isEmpty(categoryVOS)) {
-            return false;
-        }
 
         Set<String> requestArchitectures = categoryVOS.stream()
                 .map(HostOsCategoryVO::getArchitecture)