qemu: Preserve original error during migration

In some cases (spotted with broken connection during tunneled migration)
we were overwriting the original error with worse or even misleading
errors generated when we were cleaning up after failed migration.

src/qemu/qemu_migration.c

@@ -1733,6 +1733,7 @@ qemuMigrationRun(struct qemud_driver *driver,
     qemuMigrationIOThreadPtr iothread = NULL;
     int fd = -1;
     unsigned long migrate_speed = resource ? resource : priv->migMaxBandwidth;
+    virErrorPtr orig_err = NULL;
 
     VIR_DEBUG("driver=%p, vm=%p, cookiein=%s, cookieinlen=%d, "
               "cookieout=%p, cookieoutlen=%p, flags=%lx, resource=%lu, "
@@ -1874,6 +1875,9 @@ qemuMigrationRun(struct qemud_driver *driver,
     ret = 0;
 
 cleanup:
+    if (ret < 0 && !orig_err)
+        orig_err = virSaveLastError();
+
     if (spec->fwdType != MIGRATION_FWD_DIRECT) {
         /* Close now to ensure the IO thread quits & is joinable */
         VIR_FORCE_CLOSE(fd);
@@ -1888,9 +1892,16 @@ cleanup:
 
     qemuMigrationCookieFree(mig);
 
+    if (orig_err) {
+        virSetError(orig_err);
+        virFreeError(orig_err);
+    }
+
     return ret;
 
 cancel:
+    orig_err = virSaveLastError();
+
     if (virDomainObjIsActive(vm)) {
         if (qemuDomainObjEnterMonitorAsync(driver, vm,
                                            QEMU_ASYNC_JOB_MIGRATION_OUT) == 0) {
@@ -2495,6 +2506,7 @@ qemuMigrationPerformJob(struct qemud_driver *driver,
     virDomainEventPtr event = NULL;
     int ret = -1;
     int resume = 0;
+    virErrorPtr orig_err = NULL;
 
     if (qemuMigrationJobStart(driver, vm, QEMU_ASYNC_JOB_MIGRATION_OUT) < 0)
         goto cleanup;
@@ -2540,6 +2552,9 @@ qemuMigrationPerformJob(struct qemud_driver *driver,
     resume = 0;
 
 endjob:
+    if (ret < 0)
+        orig_err = virSaveLastError();
+
     if (resume && virDomainObjGetState(vm, NULL) == VIR_DOMAIN_PAUSED) {
         /* we got here through some sort of failure; start the domain again */
         if (qemuProcessStartCPUs(driver, vm, conn,
@@ -2569,6 +2584,11 @@ endjob:
         vm = NULL;
     }
 
+    if (orig_err) {
+        virSetError(orig_err);
+        virFreeError(orig_err);
+    }
+
 cleanup:
     if (vm)
         virDomainObjUnlock(vm);