Ah yes, I always forget about AppArmor… indeed the file load was denied.
[334537.614355] audit: type=1400 audit(1655658464.067:39848): apparmor="DENIED" operation="open" profile="lxd-fedora-coreos_</var/snap/lxd/common/lxd>" name="/var/lib/snapd/hostfs/lxd/tmp/default.ign" pid=461143 comm="qemu-system-x86" requested_mask="r" denied_mask="r" fsuid=0 ouid=1000
It works way better now with the raw.apparmor option :
$ lxc launch fcos/36 fedora-coreos -p vm \
-c raw.qemu="-fw_cfg name=opt/com.coreos/config,file=/var/lib/snapd/hostfs/lxd/tmp/default.ign" \
-c raw.apparmor="/var/lib/snapd/hostfs/lxd/tmp/default.ign r,"
And if I try to connect to the instance, it works as expected :
$ ssh core@10.0.122.120
Fedora CoreOS 36.20220522.3.0
Tracker: https://github.com/coreos/fedora-coreos-tracker
Discuss: https://discussion.fedoraproject.org/tag/coreos
[core@fcos-lxd ~]$
Thank you @stgraber !