Add openstack-xapi-plugins RPM

OpenStack uses a few XAPI plugins to control XenServer.   There is a spec for building an RPM of these plugins, described here:

    https://github.com/openstack/nova/tree/master/plugins/xenserver/xenapi

This spec differs slightly:
    The name is 'openstack-xapi-plugins', which is more accurate than 'openstack-xen-plugins'
    The release used is slightly newer
    The group is 'System/Hypervisor', which seems more appropriate than 'Applications/Utilities'
    The spec file is a bit shorter

Depending on how widespread the OpenStack RPM is, it may be more prudent to switch back to its name and group.