Mingw32 complains if you request export of a symbol which does
not in fact exist.

* src/libvirt_bridge.syms, src/libvirt_macvtap.syms: Delete
  obsolete files
* src/libvirt_private.syms: Remove virNetServerGetDBusConn
* src/libvirt_dbus.syms: Add virNetServerGetDBusConn