Add a Unix domain sockets EVENTS implementation

Every listening event channel becomes a listening Unix domain socket
in a directory which can be configured via the "XEN_ROOT" environment
variable.

Every event becomes a single byte write picked up by a receiving thread
which increments the counter and broadcasts on the condition variable.

Signed-off-by: David Scott <dave.scott@citrix.com>
In_memory: when writing Configuration, broadcast

Without this a client blocked in read never wakes up.

Signed-off-by: David Scott <dave.scott@citrix.com>