scsi: pvscsi: check command descriptor ring buffer size (CVE-2016-4952)

Vmware Paravirtual SCSI emulation uses command descriptors to
process SCSI commands. These descriptors come with their ring
buffers. A guest could set the ring buffer size to an arbitrary
value leading to OOB access issue. Add check to avoid it.

Reported-by: Li Qiang <liqiang6-s@360.cn>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Cc: qemu-stable@nongnu.org
Message-Id: <1464000485-27041-1-git-send-email-ppandit@redhat.com>
Reviewed-by: Shmulik Ladkani <shmulik.ladkani@ravellosystems.com>
Reviewed-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
