root@klin:/btrfs# kvm-nbd --port 10000 212052.qcow2 &

root@klin:/btrfs# nbd-client localhost 10000 /dev/nbd0
Negotiation: ..size = 52428800KB
bs=1024, sz=52428800

root@klin:/btrfs# mount /dev/nbd0p1 /mnt/tmp
root@klin:/btrfs# df /mnt/tmp
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/nbd0p1           51605436     53064  48930968   1% /mnt/tmp