Current buffer size fails the assersion check in like

    hw/scsi/scsi-bus.c:1655:    assert(req->sense_len <= sizeof(req->sense));

when backend (block/iscsi.c) returns more data then 96.

Exercise the core dump path by booting an Gentoo ISO with scsi-generic
device backed with iscsi (built with libiscsi 1.7.0):

    x86_64-softmmu/qemu-system-x86_64 \
    -drive file=iscsi://localhost:3260/iqn.foobar/0,if=none,id=drive-disk \
    -device virtio-scsi-pci,id=scsi1,bus=pci.0,addr=0x6 \
    -device scsi-generic,drive=drive-disk,bus=scsi1.0,id=iscsi-disk \
    -boot d \
    -cdrom gentoo.iso

    qemu-system-x86_64: hw/scsi/scsi-bus.c:1655: scsi_req_complete:
    Assertion `req->sense_len <= sizeof(req->sense)' failed.

According to SPC-4, section 4.5.2.1, 252 is the limit of sense data. So
increase the value to fix it.

Also remove duplicated define for the macro.
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NBenoit Canet <benoit@irqsave.net>
Cc: qemu-stable@nongnu.org
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>