vhost: fix messages results handling

vhost: fix messages results handling

Return of message handling has now changed to an enum that can
take non-negative value that is not zero in case a reply is
needed. But the code checking the variable afterwards has not
been updated, leading to success messages handling being
treated as errors.

External post and pre callbacks return type needs also to be
changed to the new enum, so that its handling is consistent.
This is done in this patch alongside with the convertion of
its only user, vhost-crypto backend.

Fixes: 0bff510b5ea6 ("vhost: unify message handling function signature")

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>