The implemented architecture fallback mechanism is not transparent to the user

lxd git-0c77815 22946 (latest/edge) is also broken.

This is normal behavior when no image can be found for the local architecture.

images: doesn't have any riscv64 images.
ubuntu: only has 21.10
ubuntu-daily: has 22.04

It is not the download of the image that is failing:

$ lxc launch images:ubuntu/21.10 ubuntu-container
Creating ubuntu-container
Error: Failed instance creation: Failed creating instance record: Requested architecture isn't supported by this host

You get a different error if the image does not exist:
Error: Failed instance creation: Failed getting remote image info: Failed getting image: The requested image couldn't be found

`images:` never had any riscv64 image. When that happens, the image selector downloads the closest image available which in this case is an amd64 image.

When an architecture is specified the expected error is thrown:

lxc launch images:ubuntu/21.10/riscv64 ubuntu-container
Creating ubuntu-container
Error: Failed instance creation: Failed getting remote image info: Failed getting image: The requested image couldn't be found

But the following still needs to be addressed.

The implemented architecture fallback mechanism is not transparent to the user.
* lxc should notify the user.
* A fallback should only occur if the foreign architecture is supported.

Status changed to 'Confirmed' because the bug affects multiple users.

+1 @xypron - it fallbacks to x86_64 and downloads the image if the riscv64 image is not found:

ubuntu@itrue-riscv64:~$ sudo lxc launch ubuntu:20.04 focal
Creating focal
Error: Failed instance creation: Failed creating instance record: Requested architecture isn't supported by this host
ubuntu@itrue-riscv64:~$ sudo lxc image list
+-------+--------------+--------+---------------------------------------------+--------------+-----------+----------+-----------------------------+
| ALIAS | FINGERPRINT | PUBLIC | DESCRIPTION | ARCHITECTURE | TYPE | SIZE | UPLOAD DATE |
+-------+--------------+--------+---------------------------------------------+--------------+-----------+----------+-----------------------------+
| | 4ca869945db4 | no | ubuntu 20.04 LTS amd64 (release) (20221018) | x86_64 | CONTAINER | 399.15MB | Nov 8, 2022 at 2:07pm (UTC) |
+-------+--------------+--------+---------------------------------------------+--------------+-----------+----------+-----------------------------+

Running "launch" first downloads the x86_64 image, then reports that it's not compatible.