libvirt.tests: Add virsh itself command test

This PR is split from:
https://github.com/autotest/virt-test/pull/1379

v1:
Add test case for command 'cd', 'pwd', 'echo', 'exit' and 'quit' in
virsh interactive mode.

v2:
Add readonly support

v3 - v4:
Change to use virshPersistent and wrapped command functions in virsh

Signed-off-by: Wayne Sun <gsun@redhat.com>