block/crypto: Simplify block_crypto_{open,create}_opts_init()

block_crypto_open_opts_init() and block_crypto_create_opts_init()
contain a virtual visit of QCryptoBlockOptions and
QCryptoBlockCreateOptions less member "format", respectively.

Change their callers to put member "format" in the QDict, so they can
use the generated visitors for these types instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>