virsh: Fix update dargs before call reference_function in VirshClosure

The dict_like_instance supposed be updated with dargs, and then
call the reference_function use the dict_like_instance, which is
the opposite now. With this fix, new_dargs could be updated with
given dargs before call the reference_function.

Signed-off-by: Wayne Sun <gsun@redhat.com>
virsh: Add virsh itself group commands

Add functions for command 'cd', 'pwd', 'echo', 'exit' and 'quit'.

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 - v4:
Change to use virshPersistent and wrapped command functions in virsh

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