xen: re-name XenDevice to XenLegacyDevice...

xen: re-name XenDevice to XenLegacyDevice...

...and xen_backend.h to xen-legacy-backend.h

Rather than attempting to convert the existing backend infrastructure to
be QOM compliant (which would be hard to do in an incremental fashion),
subsequent patches will introduce a completely new framework for Xen PV
backends. Hence it is necessary to re-name parts of existing code to avoid
name clashes. The re-named 'legacy' infrastructure will be removed once all
backends have been ported to the new framework.

This patch is purely cosmetic. No functional change.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Anthony Perard <anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>