tests: install "qemu" namespace package into venv

tests: install "qemu" namespace package into venv

This patch adds the "qemu" namespace package to the $build/tests/venv
directory. It does so in "editable" mode, which means that changes to
the source python directory will actively be reflected by the venv.

This patch also then removes any sys.path hacking from the avocado test
scripts directly. By doing this, the environment of where to find these
packages is managed entirely by the virtual environment and not by the
scripts themselves.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220526000921.1581503-7-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>