qemu-img: Make MapEntry a QAPI struct

The "flags" bit mask is expanded to two booleans, "data" and "zero";
"bs" is replaced with "filename" string.

Refactor the merge conditions in img_map() into entry_mergeable().

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1453780743-16806-16-git-send-email-famz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>