storage: iscsi: ensure service directory exists

while creating symlink to the device, a check on existing directory
was missing, causing symlink to fail.
Added missing directory creation and some debug code.

Change-Id: Ia1d4783437236b495b2ca5424185aeac9eb8ba72
Bug-Url: https://bugzilla.redhat.com/1099882
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>
Signed-off-by: Jiri Moskovcak <jmoskovc@redhat.com>