scsi: virtio_scsi: implement request batching

Adding the command and kicking the virtqueue so far was done one after
another.  Make the kick optional, so that we can take into account
SCMD_LAST.  We also need a commit_rqs callback to kick the device if
blk-mq aborts the submission before the last request is reached.

Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>