Fix memory handling of 'call' in ovirt_rest_call_async() calls

ovirt_rest_call_async() will end up calling rest_proxy_call_async()
which takes a reference on its RestProxyCall argument so we can
drop ours after calling ovirt_rest_call_async()
This fixes a memory leak reported by valgrind.
Don't leak old OvirtCdrom::file value when setting it

Leak reported by valgrind