Okay, I misunderstood how the import command works. You need to specify the poolname even if it is in the label:
# zpool import -d /var/lib/zfs_img/ zfs_lxd
# zpool status
pool: zfs_lxd
state: ONLINE
status: Some supported features are not enabled on the pool. The pool can
still be used, but some features are unavailable.
action: Enable all features using 'zpool upgrade'. Once this is done,
the pool may no longer be accessible by software that does not support
the features. See zpool-features(5) for details.
scan: resilvered 45K in 00:00:00 with 0 errors on Thu Aug 19 07:00:27 2021
config:
NAME STATE READ WRITE CKSUM
zfs_lxd ONLINE 0 0 0
mirror-0 ONLINE 0 0 0
/var/lib/zfs_img/zfs0.img ONLINE 0 0 0
/var/lib/zfs_img/zfs1.img ONLINE 0 0 0
mirror-1 ONLINE 0 0 0
/var/lib/zfs_img/zfs2.img ONLINE 0 0 0
/var/lib/zfs_img/zfs3.img ONLINE 0 0 0
errors: No known data errors
# lxc list
Error: Get "http://unix.socket/1.0": dial unix /var/lib/lxd/unix.socket: connect: no such file or directory
# rc-service lxd status
* status: crashed
# rc-service lxd restart
* Stopping lxd service (but not containers) ...
* start-stop-daemon: no matching processes found [ ok ]
* Starting lxd service ...
# rc-service lxd status
* status: started