qemu_ga: Don't overwrite errors on FSThaw

We can tell qemuDomainSnapshotFSThaw if we want it to report errors or
not. However, if we don't want to and an error has been already set by
previous qemuReportError() we must keep copy of that error not just a
pointer to it. Otherwise, it get overwritten if FSThaw reports an error.
(cherry picked from commit 650da0e9)

src/qemu/qemu_driver.c

@@ -9636,16 +9636,13 @@ qemuDomainSnapshotFSThaw(struct qemud_driver *driver,
 
     qemuDomainObjEnterAgent(driver, vm);
     if (!report)
-        err = virGetLastError();
+        err = virSaveLastError();
     thawed = qemuAgentFSThaw(priv->agent);
-    if (!report) {
-        if (err)
-            virResetError(err);
-        else
-            virResetLastError();
-    }
+    if (!report)
+        virSetError(err);
     qemuDomainObjExitAgent(driver, vm);
 
+    virFreeError(err);
     return thawed;
 }