src: add support for g_autoptr with virObject instances

src: add support for g_autoptr with virObject instances

Libvirt currently uses the VIR_AUTOUNREF macro for auto cleanup of
virObject instances. GLib approaches things differently with GObject,
reusing their g_autoptr() concept.

This introduces support for g_autoptr() with virObject, to facilitate
the conversion to GObject.

Only virObject classes which are currently used with VIR_AUTOREF are
updated. Any others should be converted to GObject before introducing
use of autocleanup.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>