Adjust the libvirt config classes' API contract for parsing
Adjust the libvirt config classes' API contract for parsing

Don't not automatically invoke the 'parse_dom' method from
the LibvirtConfigObject constructor, because this is too
early in construction - no child class constructors have
initialized yet.

Instead require the 'parse_dom' method to be invoked after
the bare object has been constructed. Also make the default
impl of 'parse_dom' take an etree.Element instance and
validate its root element name. Add a convenient 'parse_str'
method for turning a XML string into an etree.Element
instance & parsing it.

Change-Id: I7eda2b59869f3b4ef9dce1602cbb05f7662f4528
Signed-off-by: Daniel P. Berrange <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

n0ano/gantt,n0ano/gantt