target: go through normal processing for zero-length REQUEST_SENSE

Now that spc_emulate_request_sense has been taught to process zero-length
REQUEST SENSE correctly, drop the special handling of unit attention
conditions from transport_generic_new_cmd.  However, for now REQUEST SENSE
will be the only command that goes through emulation for zero lengths.

(nab: Fix up zero-length check in transport_generic_new_cmd)

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>