doc: Update user guide

The main changes are as below:

* Add commands to test Nginx web server and rpmsg-demo, both manually
  and using autorun.
* Running virtualization configuration will not connect to Linux
  terminal by default.
* Add explanation about XEN_DOM0LESS_DOM_LINUX_DEMO_AUTORUN and
  TESTIMAGE_AUTO
* Update validation output examples.

Signed-off-by: Qi Feng <qi.feng@arm.com>
Issue-Id: SCM-4879
Change-Id: I69b54898f196c22793ce6263a276168acda662cb