scsi: fix accounting of writes

scsi: fix accounting of writes

Writes go through scsi_write_complete at least twice, the first time
to get some data without having actually written anything.  Because
of this, the first time scsi_write_complete is called it will call
bdrv_acct_done and account a read incorrectly.  Fix this by looking
at the aiocb.  I am doing the same in scsi_read_complete for symmetry,
but it is only needed in the (bogus) case of bdrv_aio_readv returning
NULL.

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