vhost/scsi: increase VHOST_SCSI_PREALLOC_PROT_SGLS to 2048

The current value of VHOST_SCSI_PREALLOC_PROT_SGLS is too small to
accommodate larger I/Os, e.g. 16-32 MiB, when the VIRTIO_SCSI_F_T10_PI
feature bit is negotiated and the backing store supports T10 PI.

vhost-scsi rejects the command with errors like:

[   59.581317] vhost_scsi_calc_sgls: requested sgl_count: 1820 exceeds pre-allocated max_sgls: 512

Signed-off-by: Greg Edwards <gedwards@ddn.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>