scsi-disk: more assertions and resets for aiocb

Leaving the aiocb to a non-NULL value leads to an assertion failure when
rerror/werror are set to stop or enospc, and the operation is retried.
scsi-disk checks that the aiocb member is NULL before filling it.

This patch correctly resets the aiocb to NULL values everywhere,
and adds the dual assertion that the aiocb was non-NULL before
calling bdrv_acct_done.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
