test(backup): mock returns no-backing-chain for rsync-failure test

Phase 6 added a hasBackingChain() check before rsync that uses
qemu-img info to detect chained incrementals. The existing
testExecuteWithRsyncFailure test mocks Script.runSimpleBashScriptForExitValue
to return 0 for any command, so the new qemu-img info check
incorrectly evaluates as "has backing chain" and routes the test
through the chain-flatten path instead of rsync — the test then
asserts a failure that never occurs.

Add a clause to the mock that returns 1 (no backing chain) for the
qemu-img info backing-filename probe, so the test continues to
exercise the rsync path it was designed for.

plugins/hypervisors/kvm/src/test/java/com/cloud/hypervisor/kvm/resource/wrapper/LibvirtRestoreBackupCommandWrapperTest.java

@@ -407,6 +407,8 @@ public class LibvirtRestoreBackupCommandWrapperTest {
                                 return 0; // File exists
                             } else if (command.contains("qemu-img check")) {
                                 return 0; // File is valid
+                            } else if (command.contains("qemu-img info") && command.contains("backing-filename")) {
+                                return 1; // No backing chain — exercise the rsync path (full backups)
                             }
                             return 0; // Other commands success
                         });