Following the renaming of the bridge management APIs, we can now
split the source file into 3 corresponding pieces

 * src/util/virnetdev.c: APIs for any type of network interface
 * src/util/virnetdevbridge.c: APIs for bridge interfaces
 * src/util/virnetdevtap.c: APIs for TAP interfaces

* src/util/virnetdev.c, src/util/virnetdev.h,
  src/util/virnetdevbridge.c, src/util/virnetdevbridge.h,
  src/util/virnetdevtap.c, src/util/virnetdevtap.h: Copied
  from bridge.{c,h}
* src/util/bridge.c, src/util/bridge.h: Split into 3 pieces
* src/lxc/lxc_driver.c, src/network/bridge_driver.c,
  src/openvz/openvz_driver.c, src/qemu/qemu_command.c,
  src/qemu/qemu_conf.h, src/uml/uml_conf.c, src/uml/uml_conf.h,
  src/uml/uml_driver.c: Update #include directives