Feature: allow users to specify kernel modules directory when running with custom kernel

[brb]

At the time of writing, the LVH runner allows users to use a custom kernel path (to bzImage) via --kernel cmdline param. Unfortunately, it doesn't allow users to specify kernel modules directory. The pre-installed kmods in a VM image usually cannot be used with the custom kernel.

One workaround is to mount linux dir via --host-mount, and then from the VM cd /host/linux && make module_install. However, having the explicit kmods param would make the LVH runner easier to use.