fixup! virt-libvirt: Implement libvirt_xml.devices.disk

Signed-off-by: Chris Evich <cevich@redhat.com>
virt-libvirt: Rename address device property

Signed-off-by: Chris Evich <cevich@redhat.com>
fixup! virt-libvirt: Add libvirt_xml nested accessor generator

Signed-off-by: Chris Evich <cevich@redhat.com>
fixup! virt-libvirt: Add libvirt_xml nested accessor generator

Signed-off-by: Chris Evich <cevich@redhat.com>
virt-libvirt: XMLAttribute accessor behavior fix

All other "get" accessors raise xcepts.LibvirtXMLNotFoundError when
their data doesn't exist.  If this returns None, then caller may
mistake the value as None, when there is no value.

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