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

Signed-off-by: Yanbing Du <ydu@redhat.com>
I'm not sure if there is a algorithm may cover these 2 judgements, I expect testers can pass a Number to this testing execpt for 2 and 3? I mean if we can export 'expect_require_features' variable to tester in test configuration, or can we improve testing to support this? it's ineed hard to know 2 and 3 includes '==', '!=' '>' and '<' meaning and comparasion.