[libvirt] [sandbox][PATCH] Add module directory prefix selection feature

Add module directory prefix selection feature This is useful when running as a non-privileged user if we want to boot a custom compiled kernel: we might not have rights to install in /lib/modules/<kernel release> so when compiling the kernel we can use "make modules_install INSTALL_MOD_PATH=/path" which installs in /path/lib/modules/<kernel release>. By setting with gvir_sandbox_config_set_moddirprefix(cfg, "/path") we can now achieve just that.