Read the guest ELF PT_NOTE from guest memory when fw_cfg
etc/vmcoreinfo entry provides the location, and write it as an
additional note in the dump.

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>