spapr_vscsi: Error handling fixes

We were incorrectly g_free'ing an object that isn't allocated
in one error path and failed to release it completely in another

This fixes qemu crashes with some cases of IO errors.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
