I just got a similar thing. /dev/zd16p1 is a partiton of 200Gb on a 200GB ZVOL
lxc config device add backup data unix-block path=/data source=/dev/zd16p1
In guest:
# df -h
/dev/sdc2 117G 12G 100G 10% /data
It reports the storage space of the host / partition !
I stupidly did
#mkfs.ext4 /data
It seems to have done the job, no error. Did I harm the host ?