tests: Vhost modules aren't loaded

[justxuewei]

Nydus tests complain that vhost* modules weren't loaded.

 Dec 22 02:14:42.617 INFO check CPU: x86_64
 Dec 22 02:14:42.617 INFO check kernel modules for: x86_64
 Dec 22 02:14:42.622 INFO kvm Ok
 Dec 22 02:14:42.626 INFO kvm_intel Ok
 Dec 22 02:14:42.630 WARN The kernel module `vhost` exist but is not already loaded. Try reloading it using 'modprobe vhost'
 Dec 22 02:14:42.633 WARN The kernel module `vhost_net` exist but is not already loaded. Try reloading it using 'modprobe vhost_net'
 Dec 22 02:14:42.635 WARN The kernel module `vhost_vsock` exist but is not already loaded. Try reloading it using 'modprobe vhost_vsock'

The default network backend of runtime-rs with Dragonball is vhost-net after #8609 merged. Loading the modules explicitly is required.