virsh.dumpxml: cpu features check enhancement

In the case, custom_cpu() hard code the feature name and policy to
update domain xml.  When check cpu features after run vrish dumpxml
command, we need check if the added features are supported by host
to get the expect require features.

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #312 from will-Do/dumpxml_update_cpu

virsh.dumpxml: cpu features check enhancement