xen: duplicate xen_disk.c as basis of dataplane/xen-block.c

The new xen-block XenDevice implementation requires the same core
dataplane as the legacy xen_disk implementation it will eventually replace.
This patch therefore copies the legacy xen_disk.c source module into a new
dataplane/xen-block.c source module as the basis for the new dataplane and
adjusts the MAINTAINERS file accordingly.

NOTE: The duplicated code is not yet built. It is simply put into place by
      this patch (just fixing style violations) such that the
      modifications that will need to be made to the code are not
      conflated with code movement, thus making review harder.

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>