dump: add API to write dump header

dump: add API to write dump header

the functions are used to write header of kdump-compressed format to vmcore.
Header of kdump-compressed format includes:
1. common header: DiskDumpHeader32 / DiskDumpHeader64
2. sub header: KdumpSubHeader32 / KdumpSubHeader64
3. extra information: only elf notes here

Signed-off-by: Qiao Nuohan <qiaonuohan@cn.fujitsu.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>