Implement registration of rrdd plugins by xenopsd.

Signed-off-by: John Else <john.else@citrix.com>
Add an argument to open_handle.

This is needed so that the grant reference gets passed to the
interdomain version of open_handle.

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

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

Signed-off-by: John Else <john.else@citrix.com>
Compile data sources from both Local and Interdomain plugins.

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 logging

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>