neutron-rootwrap-xen-dom0 handles data from stdin

The neutron-rootwrap-xen-dom0 script and the netwrap plugin have been
modified to pass stdin from one to the other.

Change-Id: Ie97980873ed95f2c96eb68f8de611de1a733b130
Closes-Bug: #1259748
Merge "neutron-rootwrap-xen-dom0 handles data from stdin"