scsi_target_send_command(): amend stable-1.6 port of the CVE-2013-4344 fix

The originally suggested fix for CVE-2013-4344 introduced a regression in
scsi_target_send_command() / REQUEST_SENSE; the third argument passed to
scsi_device_get_sense() -- for the "len" parameter -- ignored the
possibility of the guest SCSI driver requesting truncated (or shorter than
full) sense data.

This could result in (r->len > req->cmd.xfer) on return, which is not
valid SCSI.

The problem was addressed in the second round, and the commit on the
master branch (84642435) is correct. However the stable-1.6 branch (the
v1.6.1 release) has the original, regressive fix (commit fdcbe7d5); let's
update it.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>