Use local ZVOL when iscsiprovider is set to none.

I use this to manage my local zpool via PVE interface. Seems to work.
Eg. config

zfs: storage
        blocksize 4k
        target none
        pool storage
        content images
        iscsiprovider none
        portal localhost


Jan 30 17:48:45 h pvedaemon[52280]: <root@pam> update VM 100: -ide1 storage:100
>> zfs list
NAME                    USED  AVAIL  REFER  MOUNTPOINT
storage                 140G  2.46T  10.4M  /storage
storage/vm-100-disk-1  34.0G  2.49T   105K  -
storage/vm-100-disk-2   106G  2.56T   105K  -