Don't pass in an event channel handle in the interface

Instead we can open the event channel device ourselves when constructing
clients and servers.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Remove pointless function

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #36 from djs55/remove-evtchn-from-interface

Simplify interface, remove evtchn