dump: add API to write vmcore

dump: add API to write vmcore

Function is used to write vmcore in flatten format. In flatten format, data is
written block by block, and in front of each block, a struct
MakedumpfileDataHeader is stored there to indicate the offset and size of the
data block.

struct MakedumpfileDataHeader {
    int64_t offset;
    int64_t buf_size;
};

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