libvirt_xml.vm_xml: Separate os tag to VMOSXML and unify names

1) Use a dedicated class VMOSXML to access the os tag in VM's XML.
2) Unify class names in vm_xml.

Signed-off-by: Hao Liu <hliu@redhat.com>
Merge pull request #1790 from Hao-Liu/fix_os_xml

libvirt_xml.vm_xml: Separate os tag to VMOSXML and unified class names