[Feature request] Illumnos VM image

We already have handy VMs to build QEMU within:

$ git grep -l basevm.BaseVM
tests/vm/centos
tests/vm/fedora
tests/vm/freebsd
tests/vm/netbsd
tests/vm/openbsd
tests/vm/ubuntu.i386

It would be useful to have a illumos VM to do build testing and avoid regressions.

Suggested by Thomas Huth:
https://<email address hidden>/msg719202.html