Implement registration of rrdd plugins by xenopsd

Signed-off-by: John Else <john.else@citrix.com>
Make /local/domain/<domid>/rrd writeable for guests

Signed-off-by: John Else <john.else@citrix.com>
Add a means of deregistering interdomain RRD plugins

A plugin will be deregistered if it writes "true" to
/local/domain/<domid>/rrd/shutdown

Signed-off-by: John Else <john.else@citrix.com>