tests: qemu: Fix TCG test.

$guestfish was not defined, so the UML test/skip did not do anything.
tests: Remove tests/protocol/test-qemudie-launchfail.sh

This test has not been run since 2012 as it depends on specifics of
how supermin and new-style appliances work (see
commit 2d89aef897e633bffba1dffd600b122eacd88fc8).

This commit removes the test entirely.
Warn about large stack frames, and fix a few places with excessive stack usage.
daemon: Truncate long error messages so XDR encoding doesn't fail.
tests: Add a protocol regression test for long error messages.
valgrind: Tweak the suppression matching OCaml leak in caml_search_in_path.

In OCaml 4.02 this still leaks, but the stack trace is slightly
different so the old suppression didn't match it.