vhost-scsi: Reduce mem use by moving upages to per queue

Each worker thread can process 1 command at a time so there's no need to
allocate a upages array per cmd. This patch moves it to per queue. Even a
small device with 128 cmds and 1 queue this brings mem use for the array
from

2 MB = 8 bytes per page pointer * 2048 pointers * 128 cmds

to

16K = 8 bytes per pointer * 2048 * 1 queue

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Message-Id: <20241203191705.19431-2-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>