Getting rid of the last bits of QEMU's 'ad-hoc CI' for merges

[Peter Maydell]

We've made pretty good progress on transitioning our pre-merge CI
from running ad-hoc on machines the person doing the merge has access to
to all CI being driven by the Gitlab CI infrastructure. For this (7.1) release
cycle I think ideally we should try to get rid of the last few bits
of ad-hoc CI so that for 7.2 we are using only the gitlab CI. (This
will help in handing over merge request management to Stefan for 7.2.)

I think the last setups I have been using ad-hoc scripting for are:

(1) PPC64 big-endian Linux
(2) NetBSD (x86)
(3) OpenBSD (x86)

I think we can get away with just dropping ppc64be -- we have
coverage for it as a cross-compile setup, and hopefully the
s390x CI runner will catch the various "fails tests on big-endian host"
issues. (Alternatively if anybody has a ppc64be machine they'd like
to let us run a gitlab CI runner on, we could do that :-))

For the BSDs, the ad-hoc CI is just running the tests/vm
"make vm-build-netbsd" etc. Is there some way we can get
coverage of this into the gitlab CI setup? (I think we
have FreeBSD via Cirrus CI, so I have not listed that one.)

thanks
-- PMM