libxl: Clarify that 'xenlight' should only be used internally

libxl: Clarify that 'xenlight' should only be used internally

The libxl driver has suffered an identity crisis since its introduction.
It took on the name 'libxl' since at the time libvirt already contained
a 'xen' driver for the old Xen toolstack implementation. 'libxl' is short
for libxenlight, which is often called xenlight. Unfortunately all forms
of the name are used in the libxl driver.

The only remaining use of the 'xenlight' form is when interacting with
the host device manager, which is difficult to change since it would
cause problems when upgrading the driver.

Rename the #define to make it clear the 'xenlight' form is internal and
add a comment describing why the name exists and that its use should be
discouraged.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>