vtpmmgr: Shutdown more gracefully

vtpmmgr: Shutdown more gracefully

vtpmmgr uses the default, weak app_shutdown, which immediately calls the
shutdown hypercall.  This short circuits the vtpmmgr clean up logic.  We
need to perform the clean up to actually Flush our key out of the tpm.

Setting do_shutdown is one step in that direction, but vtpmmgr will most
likely be waiting in tpmback_req_any.  We need to call shutdown_tpmback
to cancel the wait inside tpmback and perform the shutdown.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibaut@ens-lyon.org>
Reviewed-by: Daniel P. Smith <dpsmith@apertussolutions.com>