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>