To ensure virnetprotocol.[ch] are generated before any other
files, add them to BUILT_SOURCES and MAINTAINERCLEANFILES.
At the same time, move ESX_DRIVER_GENERATED out of DISTCLEAN
and into MAINTAINERCLEANFILES, since they are included in
EXTRA_DIST

* src/Makefile.am: Add virnetprotocol.[ch] to BUILT_SOURCES