service: Fix user initiated shutdown with static tracing

Partially revert commit 272f273 to stop
a user-initiated shutdown (typing `exit` or control-d for example at a
shall prompt) from hanging and leaving the hypervisor running. This
issue only occurs when static agent tracing is in operation.

Fixes #479.

Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>

kata-agent.service.in

@@ -16,4 +16,7 @@ StandardOutput=tty
 Type=simple
 ExecStart=@bindir@/@kata-agent@
 LimitNOFILE=infinity
-ExecStop=/bin/sync
+# ExecStop is required for static agent tracing; in all other scenarios
+# the runtime handles shutting down the VM.
+ExecStop=/bin/sync ; /usr/bin/systemctl --force poweroff
+FailureAction=poweroff