ZFS: mount subvols in activate_volume

ZFS: mount subvols in activate_volume

Makes it possible to clone and start a container whose
ZFS subvols are not yet mounted for some reason. If a
subvol cannot be mounted, there's a better error now:
zfs error: cannot mount '/myzpool/subvol-103-disk-0': directory is not empty

Previously, cloning would quietly do an "empty" clone,
and startup would fail with:
mount_autodev: 1074 Permission denied - Failed to create "/dev" directory
lxc_setup: 3238 Failed to mount "/dev"
do_start: 1224 Failed to setup container "103"
__sync_wait: 41 An error occurred in another process (expected sequence number 5)

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>