virt: Add a func to get infac count of VM

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Staticmethods to get devices is a temp approach before we have libvirt_xml/devices. But now we should use it rather than add more and more staticmethods in vm_xml.py. When Chris is working for virttest/libvirt, he had complained about this a lot times.