iscsi: change the format of the 'luns' parameter

Previously, the parameter was a list of luns in <name>=<id> format, leaving no
room to specify the HBA type (iblock was assumed) and, in my opinion, in the
reverse order to what makes sense. This patch changes the format of this
parameter to be <id>:<hba>/<name>, e.g. "0:iblock/test1".

Signed-off-by: Chris Boot <crb@tiger-computing.co.uk>
iscsi: fix stop action with disabled TPGs

If an TPG exists but is already disabled (e.g. because it failed to be enabled
for whatever reason), stopping it fails. Without this commit, the stop action
attempts to disable already disabled TPGs, and doesn't attempt to disable
enabled TPGs.

Signed-off-by: Chris Boot <crb@tiger-computing.co.uk>