vhost scsi: alloc cmds per vq instead of session

We currently are limited to 256 cmds per session. This leads to problems
where if the user has increased virtqueue_size to more than 2 or
cmd_per_lun to more than 256 vhost_scsi_get_tag can fail and the guest
will get IO errors.

This patch moves the cmd allocation to per vq so we can easily match
whatever the user has specified for num_queues and
virtqueue_size/cmd_per_lun. It also makes it easier to control how much
memory we preallocate. For cases, where perf is not as important and
we can use the current defaults (1 vq and 128 cmds per vq) memory use
from preallocate cmds is cut in half. For cases, where we are willing
to use more memory for higher perf, cmd mem use will now increase as
the num queues and queue depth increases.
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Link: https://lore.kernel.org/r/1604986403-4931-3-git-send-email-michael.christie@oracle.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Maurizio Lombardi <mlombard@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>