libvirt_xml: update CPUXML and capabilityXML

Commit 393d19fa update class CPUXML without update related methods
of it, so fix it. And create a new func to do feature number check.

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #1805 from will-Do/vm_cpu_xml

libvirt_xml: update CPUXML and capabilityXML