kdump: set vmcoreinfo location

kdump header provides offset and size of the vmcoreinfo content,
append it if available (skip the ELF note header).

crash-7.1.9 was the first version that started looking in the
vmcoreinfo data for phys_base instead of in the kdump_sub_header.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
