mount /dev/sda2 /mnt
mount --bind /dev /mnt/dev
mount --bind /proc /mnt/proc
mount --bind /sys /mnt/sys
chroot /mnt
mount -a
exit ## exit from chroot environment
Those last three lines should mount all of the “btrfs” subvolumes. And, after doing that, you should be able to see your files at “/mnt/srv/www/htdocs”.