network: improve error report when firewall chain creation fails

network: improve error report when firewall chain creation fails

During startup we create some top level chains in which all
virtual network firewall rules will be placed. The upfront
creation is done to avoid slowing down creation of individual
virtual networks by checking for chain existance every time.

There are some factors which can cause this upfront creation
to fail and while a message will get into the libvirtd log
this won't be seen by users who later try to start a virtual
network. Instead they'll just get a message saying that the
libvirt top level chain does not exist. This message is
accurate, but unhelpful for solving the root cause.

This patch thus saves any error during daemon startup and
reports it when trying to create a virtual network later.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>