qemu: Remove the managed state file only if restoring succeeded

1) Both "qemuDomainStartWithFlags" and "qemuAutostartDomain" try to
restore the domain from managedsave'ed image if it exists (by
invoking "qemuDomainObjRestore"), but it unlinks the image even
if restoring fails, which causes data loss. (This problem exists
for "virsh managedsave dom; virsh start dom").

The fix for is to unlink the managed state file only if restoring
succeeded.

2) For "virsh save dom; virsh restore dom;", it can cause data
corruption if one reuse the saved state file for restoring. Add
doc to tell user about it.

3) In "qemuDomainObjStart", if "managed_save" is NULL, we shouldn't
fallback to start the domain, skipping it to cleanup as a incidental
fix. Discovered by Eric.

src/qemu/qemu_driver.c

@@ -3423,18 +3423,20 @@ static int qemudDomainObjStart(virConnectPtr conn,
 
     /*
      * If there is a managed saved state restore it instead of starting
-     * from scratch. In any case the old state is removed.
+     * from scratch. The old state is removed once the restoring succeeded.
      */
     managed_save = qemuDomainManagedSavePath(driver, vm);
+
+    if (!managed_save)
+        goto cleanup;
+
     if ((managed_save) && (virFileExists(managed_save))) {
         ret = qemuDomainObjRestore(conn, driver, vm, managed_save);
 
-        if (unlink(managed_save) < 0) {
+        if ((ret == 0) && (unlink(managed_save) < 0))
             VIR_WARN("Failed to remove the managed state %s", managed_save);
-        }
 
-        if (ret == 0)
-            goto cleanup;
+        goto cleanup;
     }
 
     ret = qemuProcessStart(conn, driver, vm, NULL, start_paused, -1, NULL,

tools/virsh.pod

@@ -546,7 +546,11 @@ I<on_reboot> parameter in the domain's XML definition.
 
 =item B<restore> I<state-file>
 
-Restores a domain from an B<virsh save> state file.  See I<save> for more info.
+Restores a domain from an B<virsh save> state file. See I<save> for more info.
+
+B<Note>: To avoid corrupting file system contents within the domain, you
+should not reuse the saved state file to B<restore> unless you are convinced
+with reverting the domain to the previous state.
 
 =item B<save> I<domain-id> I<state-file>