rpc: refactor virNetServer setup for post-exec restarts

rpc: refactor virNetServer setup for post-exec restarts

With the current code it is neccessary to call

  virNetDaemonNewPostExecRestart()

and then for each server that needs restarting you are supposed
to call

  virNetDaemonAddSeverPostExecRestart()

This is fine if there's only ever one server, but as soon as you
have two servers it is impossible to use this design. The code
has no idea which servers were recorded in the JSON state doc,
nor in which order the hash table serialized its keys.

So this patch changes things so that we only call

  virNetDaemonNewPostExecRestart()

passing in a callback, which is invoked once for each server
found int he JSON state doc.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>