scsi-generic: Handle canceled request in scsi_command_complete

Now that we always called the cb in bdrv_aio_cancel, let's make scsi-generic
callbacks check io_canceled flag similarly to scsi-disk.

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