Prior to this patch we didn't make any attempt to prevent two entries
in the array of interfaces/PCI devices from pointing to the same
device.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1002423

This patch introduces the new forward mode='hostdev' along with
attribute managed. Includes updates to the network RNG and new xml
parser/formatter code.
Signed-off-by: NShradha Shah <sshah@solarflare.com>