[SCSI] libiscsi: make sure session is not blocked when removing host

When we logout we block the session since we are not taking any more
commands, but when we call remove host we want to make sure any
IO that got queued up and blocked gets failed upwards quickly, so
we unblock the session and fail it.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>