fix regression in zfs volume activation

fix regression in zfs volume activation

commit 815df2dd08ac4c7295135262e60d64fbb57b8f5c introduced a small issue
when activating linked clone volumes - the volname passed contains
basevol/subvol, which needs to be translated to subvol.

using the path method should be a robust way to get the actual path for
activation.

Found and tested by building the package as root (otherwise the zfs
regressiontests are skipped).

Reported-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>