app/vfe-vdpa: add sleep interval in restoring VFs

When multiple VFs need to be restored, per-VF restore time will be
longer because of QEMU clean-up and device config takes more time
for one VF. Therefore, add a sleep interval of 4 seconds for one VF
so that downtime for one VF will be stable.

Signed-off-by: Chenbo Xia <chenbox@nvidia.com>

app/vfe-vdpa/main.c

@@ -886,6 +886,7 @@ virtio_ha_client_dev_restore(void)
 				ret = -1;
 				goto err_vf_ctx;
 			}
+			sleep(4);
 err_vf_ctx:
 			free(vf_ctx);
 		}