5.15+ GCE Instances are unable to create fb0 with virt mon attached

This is a public facing LP issue to address the content in Canonical's SF#00366439.

Summary:

Post 18.04 ( or 20.04 prior to moving to 5.15 ), efifb fails to stand up fb0. It is assigned, but fb0 is never created, nor are there any errors that I noted. Here is an example from a focal using linux-gcp
---
5.15.0-1041-gcp #49~20.04.1-Ubuntu SMP
kyler_hornor@kyler-focal:~$ sudo dmesg | grep efifb
[ 0.925827] pci 0000:00:05.0: BAR 0: assigned to efifb
---

If you then install the generic HWE kernel and reboot, you can see that fb0 is successfully summoned:

---
5.15.0-83-generic #92~20.04.1-Ubuntu SMP
kyler_hornor@kyler-focal:~$ sudo dmesg | grep efifb
[ 2.128815] pci 0000:00:05.0: BAR 0: assigned to efifb
[ 3.021819] efifb: probing for efifb
[ 3.022511] efifb: framebuffer at 0xc0000000, using 6076k, total 6075k
[ 3.023925] efifb: mode is 1920x1080x24, linelength=5760, pages=1
[ 3.025382] efifb: scrolling: redraw
[ 3.026347] efifb: Truecolor: size=0:8:8:8, shift=0:16:8:0
---

and finally, using linux-gcp-lts on 20.04, it does work (as it's 5.4):
---
5.4.0-1112-gcp #121-Ubuntu SMP
kyler_hornor@kyler-focal:~$ sudo dmesg | grep efifb
[ 0.834541] pci 0000:00:05.0: BAR 0: assigned to efifb
[ 1.057258] efifb: probing for efifb
[ 1.058182] efifb: framebuffer at 0xc0000000, using 6076k, total 6075k
[ 1.059210] efifb: mode is 1920x1080x24, linelength=5760, pages=1
[ 1.060203] efifb: scrolling: redraw
[ 1.060722] efifb: Truecolor: size=0:8:8:8, shift=0:16:8:0
---

So some linux-gcp backport or config seems to be breaking this.

Supplementary to this, the framebuffer can not be leveraged by X and throws a fatal error around the time shadow is loaded. This part is replicable on -generic, as well as on debian using gdm3/X, so this is likely an upstream problem. I tried disabling shadow with an X conf, but as it uses 24bppp, it forces it.

For the scope of this LP, we need to figure out the -gcp specific breakage. I've reached out to google to see if we can get some contacts on the virt mon team to push the other aspect forward.

Replication:
Launch any ubuntu 20.04+ GCE instance with the "Display Device" option enabled.

Expected Behavior:
We should see the above output in dmesg and also observe /dev/fb0 being created.