fix #2120: use hosts initiator name with qemu-img

fix #2120: use hosts initiator name with qemu-img

qemu-img uses the qemu default initiator name 'iqn.2008-11.org.linux-kvm'
since we use the one of the host (/etc/iscsi/initiatorname.iscsi) when
using it with a running vm, we want to using it also when moving a disk
with qemu-img

to do that we have give qemu-img the image in as a full option string

this fixes the issue that we could not move an zfs-over-iscsi disk
without allowing the default qemu initiator

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>