lxd/storage/drivers/driver/zfs/volumes: Include unmount action in the revert hook returned from CreateVolumeFromBackup
lxd/storage/drivers/driver/zfs/volumes: Include unmount action in the revert hook returned from CreateVolumeFromBackup

This way if a failure occurs after CreateVolumeFromBackup is finished, and its returned revert hook is run, the volume's internal mount
counter is correctly cleared.

This prevents future import attempts from failing the post import volume resize due to In Use errors caused by the left over mount counter.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

stgraber/lxd,hallyn/lxd,hallyn/lxd,atxwebs/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,atxwebs/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,atxwebs/lxd,lxc/lxd,atxwebs/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd