img_convert has been around before there was an ImgConvertState or
a block backend, but it has never been modified to directly use
these structs. Change this by parsing parameters directly into
the ImgConvertState and directly use BlockBackend where possible.
Furthermore variable initialization has been reworked and sorted.
Signed-off-by: NPeter Lieven <pl@kamp.de>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>