proxy: Update OvirtVmDisplay::ca-cert when list of VM changes
proxy: Update OvirtVmDisplay::ca-cert when list of VM changes

In order to achieve that, when OvirtProxy::api is set, we start watching
for changes on the OvirtApi::vms collection through notify::resources, and
update the new OvirtVmDisplay when this is triggered.

etrunko/libgovirt,GNOME/libgovirt,etrunko/libgovirt