Ignore xenops-cli autogen code from ocamllex/yacc

Remove autogenerated files from git, have make rule clean them up and add them
to .gitignore.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #1488 from simonjbeaumont/ignore-autogen

Ignore xenops-cli autogen code from ocamllex/yacc