Fix ovirt_collection_fetch_finish() annotation
proxy: Move tmp file writing code to a separate function

g_file_new_tmp is not available on older glib, so we need a fallback
for these. Having a separate function handling tmp file writing
is the first step towards that.
Add fallback write_to_tmp_file() for older glibs

Since it's using file descriptors, it's likely not to work as
expected on Windows, that's why I'm keeping the gio based version.
Fix setting of OvirtProxy::ssl-ca-file from tmp file

We were setting the path for the temporary file using g_object_set,
which triggered a callback listening for ssl-ca-file changes, which
made sure the temp file was removed when ssl-ca-file is modified.
We obviously do not want to do that when we are currently setting
the path to the temporary file we just created, so we just block
this handler while setting it.
Remove 2 debug g_warning()

They are causing spurious logs in normal use.
Fix leak in ovirt_proxy_get_collection_xml_async()

ovirt_rest_call_async will take a reference on 'call' when needed,
so we can release the reference we got through ovirt_rest_call_new()
after calling ovirt_rest_call_async()