Revert "tests: Use separate virtual environment for avocado"

This reverts commit e8e4298fea.

ensuregroup allows to specify both the acceptable versions of avocado,
and a locked version to be used when avocado is not installed as a system
pacakge.  This lets us install avocado in pyvenv/ using "mkvenv.py" and
reuse the distro package on Fedora and CentOS Stream (the only distros
where it's available).

ensuregroup's usage of "(>=..., <=...)" constraints when evaluating
the distro package, and "==" constraints when installing it from PyPI,
makes it possible to avoid conflicts between the known-good version and
a package plugins included in the distro.

This is because package plugins have "==" constraints on the version
that is included in the distro, and, using "pip install avocado==88.1"
on a venv that includes system packages will result in an error:

   avocado-framework-plugin-varianter-yaml-to-mux 98.0 requires avocado-framework==98.0, but you have avocado-framework 88.1 which is incompatible.
   avocado-framework-plugin-result-html 98.0 requires avocado-framework==98.0, but you have avocado-framework 88.1 which is incompatible.

But at the same time, if the venv does not include a system distribution
of avocado then we can install a known-good version and stick to LTS
releases.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1663
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

pythondeps.toml

@@ -23,3 +23,10 @@ meson = { accepted = ">=0.63.0", installed = "0.63.3", canary = "meson" }
 [docs]
 sphinx = { accepted = ">=1.6", installed = "5.3.0", canary = "sphinx-build" }
 sphinx_rtd_theme = { accepted = ">=0.5", installed = "1.1.1" }
+
+[avocado]
+# Note that qemu.git/python/ is always implicitly installed.
+# Prefer an LTS version when updating the accepted versions of
+# avocado-framework, for example right now the limit is 92.x.
+avocado-framework = { accepted = "(>=88.1, <93.0)", installed = "88.1", canary = "avocado" }
+pycdlib = { accepted = ">=1.11.0" }