Qemu Test: Add a hello world test for virt tests (qemu) v3

This patch is a "Hello, world!" test for virt tests (qemu).

The goal of this script is to provide autotest newbie
an example for some most frequently used interfaces.

Changes from v2:
 * Make test structure more flat as now the framework
   closes unused ssh sessions at the end of the test
 * Use session.cmd, since both this and cmd_status_output
   will raise exceptions so they'd have to be handled
   anyway
 * Use a more customary "Hello, world!" string
 * Really verify whether the guest outputs the right
   string, to show how to fail a test.
 * Introduce checking VM state.
 * Improve the comments on code.

Changes from v1:
 * Fix a typo in commit message.

Signed-off-by: yunzheng <yunzheng@redhat.com>
Signed-off-by: Feng Yang <fyang@redhat.com>
Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>