Get XENVIF up and running.

Create a new stub EMULATED interface and make some other tweaks to get
XENVIF to come up and enumerate child devices.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Switch to test signing.

New xenbus.pfx (no password) added to repo.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>