virsh.dumpxml: Add cases for --update-cpu option

Signed-off-by: Yanbing Du <ydu@redhat.com>
In this case, I hard code the cpu new features in func ```custom_cpu()```, which result the expecte require features number.(It's not very suitable for user input)
I agree a variable like 'expect_require_features' is much better for understanding the vaule meaning, but as all the custom features are specified in ```custom_cpu()```, maybe i can set the variable in that func and give more comments.