network: Separate private APIs declaration to bridge_driver_priv.h

network: Separate private APIs declaration to bridge_driver_priv.h

There are two functions implemented in bridge_driver.c that are
used in tests (networkDnsmasqCreateXMLConf() and
networkDnsmasqConfContents()) but are declared in
bridge_driver.h. This goes against our current practice where
such APIs are declared in $name_priv.h.

Therefore, move those APIs to bridge_driver_priv.h

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>