xenserver-core: Don't depend on openstack-xapi-plugins

Currently we package plugins for the latest stable OpenStack
release.   This causes problems for people who want to run
a development version.   We should not install this package
by default, but keep it available for people who would like
to install the stable OpenStack release.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #208 from euanh/build-improvements

xenserver-core: Don't depend on openstack-xapi-plugins