tcmu: Support emulate_write_cache

This will enable the toggling of write_cache in tcmu through targetcli-fb

Signed-off-by: Bryant G. Ly <bryantly@linux.vnet.ibm.com>
Reviewed-By: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>