virsh: Fix update dargs before call reference_function in VirshClosure

Update dargs with dict_like_instance but not overwrite values in
dargs, then call the reference_function use dargs.

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>