Remove incorrectly used TODO macro

Remove incorrectly used TODO macro

The TODO macro expands to an fprintf() call and is used in several
places in the Xen driver. Anything that wishes to print such debug
messages should use the logging macros. In this case though, all the
places in the Xen driver should have been raising a formal libvirt
error instead. Add proper error handling and delete the TODO macro
to prevent future misuse.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>