Comment 4 for bug 1639878

The initramfs with block devices only are fine.
But I'd like to point out in case that you're not aware, the only drivers you have are below. That is probably only going to work for kvm and probably not all the virtual devices it can present as root. I doubt almost any hardware system would boot.

   kernel/fs/nls/nls_iso8859-1.ko
   kernel/ubuntu/i915/i915_bpo.ko
   kernel/drivers/platform/x86/intel_ips.ko
   kernel/drivers/platform/x86/mxm-wmi.ko
   kernel/drivers/platform/x86/wmi.ko
   kernel/drivers/i2c/algos/i2c-algo-bit.ko
   kernel/drivers/scsi/virtio_scsi.ko
   kernel/drivers/char/agp/sis-agp.ko
   kernel/drivers/iommu/amd_iommu_v2.ko
   kernel/drivers/video/vgastate.ko
   kernel/drivers/video/fbdev/core/syscopyarea.ko
   kernel/drivers/video/fbdev/core/fb_sys_fops.ko
   kernel/drivers/video/fbdev/core/sysfillrect.ko
   kernel/drivers/video/fbdev/core/sysimgblt.ko
   kernel/drivers/video/fbdev/vga16fb.ko
   kernel/drivers/video/fbdev/sis/sisfb.ko
   kernel/drivers/mmc/card/mmc_block.ko
   kernel/drivers/mmc/host/sdhci.ko
   kernel/drivers/mmc/host/sdhci-acpi.ko
   kernel/drivers/mmc/host/sdhci-pci.ko
   kernel/drivers/gpu/drm/nouveau/nouveau.ko
   kernel/drivers/gpu/drm/i2c/sil164.ko
   kernel/drivers/gpu/drm/i2c/tda998x.ko
   kernel/drivers/gpu/drm/i2c/ch7006.ko
   kernel/drivers/gpu/drm/amd/amdkfd/amdkfd.ko
   kernel/drivers/gpu/drm/amd/amdgpu/amdgpu.ko
   kernel/drivers/gpu/drm/qxl/qxl.ko
   kernel/drivers/gpu/drm/r128/r128.ko
   kernel/drivers/gpu/drm/via/via.ko
   kernel/drivers/gpu/drm/gma500/gma500_gfx.ko
   kernel/drivers/gpu/drm/i810/i810.ko
   kernel/drivers/gpu/drm/radeon/radeon.ko
   kernel/drivers/gpu/drm/cirrus/cirrus.ko
   kernel/drivers/gpu/drm/vmwgfx/vmwgfx.ko
   kernel/drivers/gpu/drm/virtio/virtio-gpu.ko
   kernel/drivers/gpu/drm/vgem/vgem.ko
   kernel/drivers/gpu/drm/drm_kms_helper.ko
   kernel/drivers/gpu/drm/tdfx/tdfx.ko
   kernel/drivers/gpu/drm/udl/udl.ko
   kernel/drivers/gpu/drm/ttm/ttm.ko
   kernel/drivers/gpu/drm/mga/mga.ko
   kernel/drivers/gpu/drm/drm.ko
   kernel/drivers/gpu/drm/savage/savage.ko
   kernel/drivers/gpu/drm/i915/i915.ko
   kernel/drivers/gpu/drm/sis/sis.ko
   kernel/drivers/gpu/drm/ast/ast.ko
   kernel/drivers/ata/ahci.ko
   kernel/drivers/ata/libahci.ko
   kernel/drivers/acpi/video.ko
   kernel/drivers/usb/storage/usb-storage.ko