libxl: Use cooperative resume for migration

Migration code specifies the problematic non-cooperative resume mode
which is a known issue with Xen's libxl [1].  Instead, use the better
supported cooperative mode.

Without this, guests BUG() in xen_irq_resume after failing to bind
still-bound event channels.

[1] http://bugs.xenproject.org/xen/bug/30

src/libxl/libxl_migration.c

@@ -147,7 +147,7 @@ libxlDoMigrateSend(libxlDriverPrivatePtr driver,
                                xl_flags, NULL);
     if (ret != 0) {
         /* attempt to resume the domain on failure */
-        if (libxl_domain_resume(priv->ctx, vm->def->id, 0, 0) != 0) {
+        if (libxl_domain_resume(priv->ctx, vm->def->id, 1, 0) != 0) {
             VIR_DEBUG("Failed to resume domain following failed migration");
             virDomainObjSetState(vm, VIR_DOMAIN_PAUSED,
                                  VIR_DOMAIN_PAUSED_MIGRATION);