virt: Small propcan module tweak

Separated out Semi-private methods from PropcanBase class
to reduce pylint noise coming from other modules.

Signed-off-by: Chris Evich <cevich@redhat.com>
fixup! virt-libvirt: Make libvirt_xml.accessors xparent

Signed-off-by: Chris Evich <cevich@redhat.com>
virt-libvirt: Update libvirt_xml.libvirt_xml to use accessors

Signed-off-by: Chris Evich <cevich@redhat.com>
virt-libvirt: Add more unittests for libvirt_xml

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