xen: add header and build dataplane/xen-block.c

xen: add header and build dataplane/xen-block.c

This patch adds the transformations necessary to get dataplane/xen-block.c
to build against the new XenBus/XenDevice framework. MAINTAINERS is also
updated due to the introduction of dataplane/xen-block.h.

NOTE: Existing data structure names are retained for the moment. These will
      be modified by subsequent patches. A typedef for XenBlockDataPlane
      has been added to the header (based on the old struct XenBlkDev name
      for the moment) so that the old names don't need to leak out of the
      dataplane code.

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