libvirt: Fix attach-device itertools backports

Calling backported itertools using 'from ... import ...' will cause
following error.

ERROR| Traceback (most recent call last):
ERROR|   File "virttest/standalone_test.py", line 182, in run_once
ERROR|     test_modules[t_type] = imp.load_module(t_type, f, p, d)
ERROR|   File "libvirt/tests/src/virsh_cmd/domain/virsh_attach_device.py", line 14, in <module>
ERROR|     from virttest.staging.backports.itertools import product, islice, count
ERROR| ImportError: No module named itertools

Signed-off-by: Hao Liu <hliu@redhat.com>