xen-scsifront: Add a missing call to kfree

Most error branches following the call to kmalloc contain
a call to kfree. This patch add these calls where they are
missing.

This issue was found with Hector.

Signed-off-by: Quentin Lambert <lambert.quentin@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>