When PV Grub will be used, VM initramfs should not contain /lib/modules
mounting code, as the VM root.img will already contains kernel modules.
Make it possible by splitting the module.
QubesOS/qubes-issues#1354
This is preparation for pvgrub support, where all VM kernel files will
be installed inside of VM instead of dom0.
But also the same could be used to prepare VM kernel image from any dom0
kernel.
