rpc: virnetserver: Move virNetServerCheckLimits which is static up in the file

rpc: virnetserver: Move virNetServerCheckLimits which is static up in the file

Since virNetServerAddClient checks for the limits in order to temporarily
suspend the services, thus not accepting any more clients, there is no reason
why virNetServerCheckLimits, which is only responsible for re-enabling
previously disabled services according to the limits, could not do both. To be
able to do that however, it needs to be moved up in the file since it's static
(and because it's just a helper and there's only one caller it should remain
 static).

Signed-off-by: Erik Skultety <eskultet@redhat.com>