mkvenv: use pip's vendored distlib as a fallback

mkvenv: use pip's vendored distlib as a fallback

distlib is usually not installed on Linux distribution, but it is vendored
into pip.  Because the virtual environment has pip via ensurepip, we
can piggy-back on pip's vendored version.  This could break if they move
our cheese in the future, but the fix would be simply to require distlib.

If it is debundled, as it is on msys, it is simply available directly.

Signed-off-by: John Snow <jsnow@redhat.com>
[Move to toplevel. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>