libvirt.tests: Add virsh domcontrol test case

Check domain control interface state while with or without job (dump,
save, restore, managedsave) on domain. When with job on domain, "occupied"
or "background job" is expected, "ok" is also expected since state is in
change. When without job on domain, 'ok' is expected.

In the test case, use subprocess to create job on domain, while run
domcontrol in loop to check domain control interface state, no strict
check on state as it is changing, only check the command status.

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