Add module directory 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_kmodpath(cfg, "/path/lib/modules") we can
now achieve just that.