libelf-loader: introduce elf_load_image

libelf-loader: introduce elf_load_image

Implement a new function, called elf_load_image, to perform the
actually copy of the elf image and clearing the padding.  The function
is implemented as memcpy and memset when the library is built as part
of the tools, but it is implemented as raw_copy_to_guest and
raw_clear_guest when built as part of Xen, so that it can be safely
called with an HVM style dom0.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>
Committed-by: Keir Fraser <keir@xen.org>