virttest.aexpect: Add exit_session function

In virsh session, if send command exit or quit, the session will be
closed. If use exist cmd_status_output function to send command exit
or quit, the ShellProcessTerminatedError will be raised and test
failed.
For test exit and quit cmd in virsh session, add exit_session
function which run exit command in virsh shell.

Signed-off-by: Wayne Sun <gsun@redhat.com>
libvirt.tests: Add virsh itself command test

v1:
Add test case for command 'cd', 'pwd', 'echo', 'exit' and 'quit' in
virsh interactive mode.

v2:
Add readonly support

v3:
Change to use virshPersistent

Signed-off-by: Wayne Sun <gsun@redhat.com>