virt: Modify libvirtxml class

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt: Add VMCPUXML class

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
libvirt: Add some testcase for virsh cpu-compare

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt: Add some comments, add chech_feature_name for users, fix some issues

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt-libvirt: rename libvirt_xml to capability_xml

Also one-char whitespace fix for vm_xml.py

Signed-off-by: Chris Evich <cevich@redhat.com>