ocaml: Add Guestfs.Errno submodule exposing useful raw errno numbers.

For use when calling G.last_errno.
launch: Return errno == ENOTSUP if discard is not supported.
mllib: Allow exit code to be specified when calling 'error' function.
virt-sparsify --in-place: Exit with code 3 if discard is not supported.

Also skip the test in this case instead of failing.