blkdebug: Simplify and improve filename generation

Instead of actually recreating the options from scratch, just reuse the
options given for creating the BDS, which are the configuration file
name and additional options. In case there are no additional options we
can thus create a plain filename.

This obviously results in a different output for qemu-iotest 099 which
exactly tests this filename generation. Fix it up as well.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1415697825-26678-2-git-send-email-mreitz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>