libxl: Resolve Coverity CHECKED_RETURN

Periodically my Coverity scan will return a checked_return failure
for libxlDomainShutdownThread call to libxlDomainStart. Followed the
libxlAutostartDomain example in order to check the status, emit a
message, and continue on.

src/libxl/libxl_domain.c

@@ -685,7 +685,11 @@ libxlDomainShutdownThread(void *opaque)
     }
     libxl_domain_destroy(ctx, vm->def->id, NULL);
     libxlDomainCleanupJob(driver, vm, VIR_DOMAIN_SHUTOFF_SHUTDOWN);
-    libxlDomainStart(driver, vm, 0, -1);
+    if (libxlDomainStart(driver, vm, false, -1) < 0) {
+        virErrorPtr err = virGetLastError();
+        VIR_ERROR(_("Failed to restart VM '%s': %s"),
+                  vm->def->name, err ? err->message : _("unknown error"));
+    }
 
  cleanup:
     if (vm)