Also fixed serial port configuration which was broken due to recent
change in virDomainChrDef where targetType was newly added.

* src/Makefile.am: add new files
* src/vbox/vbox_driver.c: add case for version 3.1
* src/vbox/vbox_tmpl.c: refactor common patterns into macros, support for
  version 3.1, serial port configuration fix
* src/vbox/vbox_CAPI_v3_1.h, src/vbox/vbox_V3_1.c: generated code