Functions that are not callbacks should assert that aiocb is NULL and
have a non-opaque argument (usually a pointer to SCSIDiskReq).
AIO callbacks should assert that aiocb is not NULL and take care of
calling block_acct done. They also have an opaque argument.
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>