configure: create a python venv unconditionally

configure: create a python venv unconditionally

This patch changes the configure script so that it always creates and
uses a python virtual environment unconditionally.

Meson bootstrapping is temporarily altered to force the use of meson
from git or vendored source (as packaged in our source tarballs). A
subsequent commit restores the use of distribution-vendored Meson.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230511035435.734312-16-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>