block/iscsi: Fix compilation for libiscsi 1.4.0 (API change)

block/iscsi: Fix compilation for libiscsi 1.4.0 (API change)

Function iscsi_read10_task got additional parameters starting with version
libiscsi 1.5.0.

libiscsi 1.4.0 is still widely used (Debian wheezy, jessie and other Linux
distributions currently provide packages for QEMU which use it), so we
still need support for this older API.

Reviewed-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>