xen: add a meaningful declaration of grant_copy_segment into xen_common.h

xen: add a meaningful declaration of grant_copy_segment into xen_common.h

Currently the xen_disk source has to carry #ifdef exclusions to compile
against Xen older then 4.8. This is a bit messy so this patch lifts the
definition of struct xengnttab_grant_copy_segment and adds it into the
pre-4.8 compat area in xen_common.h, which allows xen_disk to be cleaned
up.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>