oxenstored: enable domain connection indexing based on eventchn port

oxenstored: enable domain connection indexing based on eventchn port

Currently in xenstore connection database,  we use a hash table of
(domid -> connection) to store domain connections. This allows fast indexing
based on dom ids.

This patch adds another dimention of fast indexing that is based on eventchn
port number. This is useful when doing selective connection processing
based on the port numbers of incoming events.

Signed-off-by: Zheng Li <dev@zheng.li>
Reviewed-by: David Scott <dave.scott@citrix.com>