When using PVGrub it doesn't make sense to attach modules.img, since
modules are already in the VM. Initramfs there will already handle such
situation and will not try to mount it, when VM's root filesystem
already contains appropriate /lib/modules/`uname -r`.
On the other hand, error earlier when initramfs is missing. While also
not used by PV Grub, the file is always specified in libvirt config and
when missing libvirt will throw rather cryptic error message.
QubesOS/qubes-issues#1354
ba54792c83