Cannot launch armhf containers on arm64 host under noble

[Dave Jones]

Under the current noble daily server image for Raspberry Pi (arm64 architecture) I cannot launch an armhf container image using lxd from the channel 5.21/stable/ubuntu-24.04 (build 28284). It *appears* to launch successfully, and lxd reports no error yet afterward the container is stopped:

$ lxc launch ubuntu-daily:n/armhf nobletest
Creating nobletest
Starting nobletest
$ lxc list
+-----------+---------+------+------+-----------+-----------+
| NAME | STATE | IPV4 | IPV6 | TYPE | SNAPSHOTS |
+-----------+---------+------+------+-----------+-----------+
| nobletest | STOPPED | | | CONTAINER | 0 |
+-----------+---------+------+------+-----------+-----------+

In case this was an issue with the current noble daily, I also attempted launching the mantic and jammy armhf images, but with the same symptoms:

$ lxc launch ubuntu:m/armhf mantictest
Creating mantictest
Starting mantictest
$ lxc list
+------------+---------+------+------+-----------+-----------+
| NAME | STATE | IPV4 | IPV6 | TYPE | SNAPSHOTS |
+------------+---------+------+------+-----------+-----------+
| nobletest | STOPPED | | | CONTAINER | 0 |
+------------+---------+------+------+-----------+-----------+
| mantictest | STOPPED | | | CONTAINER | 0 |
+------------+---------+------+------+-----------+-----------+

I attempted the same tests under the 23.10 server for Pi images (also arm64 host) and both containers launched successfully, so this appears to be an issue with lxd under noble specifically.