Fix ovirt_xx_get_yy 'transfer' annotations

Several of these functions were documented as 'transfer full' while they
actually are 'transfer none'. Thanks to Jonathon Jongsma for pointing
this out.
Update NEWS file
Release bump

This should have been done right after the 0.3.0 release :)
Add missing initialization for GError

GError must be initialized to NULL before being valid.
Allow NULL OvirtProxy::ca-cert property

This can be used to unset a previously set certificate authority.