tools/lixenguest: hide struct elf_dom_parms layout from users

tools/lixenguest: hide struct elf_dom_parms layout from users

Don't include struct elf_dom_parms in struct xc_dom_image, but rather
use a pointer to reference it. Together with adding accessor functions
for the externally needed elements this enables to drop including the
Xen private header xen/libelf/libelf.h from xenguest.h.

Fixes: 7e0165c19387 ("tools/libxc: untangle libxenctrl from libxenguest")
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wl@xen.org>