libxl: unref objects in error paths

libxl: unref objects in error paths

libxlMakeNic opens a virConnect object and takes a reference on a
virNetwork object, but doesn't drop the references on all error
paths. Rework the function to follow the standard libvirt pattern
of using a local 'ret' variable to hold the function return value,
performing all cleanup and returning 'ret' at a 'cleanup' label.