[Libguestfs] missing btrfs subvol support

[Olaf Hering]

Is btrfs subvol support failing just for me? Looks like nothing adds the
required '@/' string. virt-ls uses the first variant of the command:

><rescue> mount -vo subvol=var/spool,ro /dev/sda2 /sysroot/
[  113.852047] BTRFS info (device sda2): disk space caching is enabled
[  113.852869] BTRFS: has skinny extents
mount: mount(2) failed: No such file or directory
><rescue> mount -vo subvol=@var/spool,ro /dev/sda2 /sysroot/
[  116.082642] BTRFS info (device sda2): disk space caching is enabled
[  116.083476] BTRFS: has skinny extents
mount: mount(2) failed: No such file or directory
><rescue> mount -vo subvol=@/var/spool,ro /dev/sda2 /sysroot/
[  123.995343] BTRFS info (device sda2): disk space caching is enabled
[  123.996238] BTRFS: has skinny extents
mount: /dev/sda2 mounted on /sysroot.
><rescue> exit

fstab is:
UUID=d538a81f-9778-424d-96cc-e48dd2d4a323 swap                 swap defaults              0 0
UUID=65b72f12-eccb-4cf2-a4f4-9c4bb462456f /                    btrfs defaults              0 0
UUID=65b72f12-eccb-4cf2-a4f4-9c4bb462456f /var/spool           btrfs subvol=@/var/spool    0 0


Olaf