vde2: build with `--disable-python` by default

This package explicitly depends on `python2` which will be EOLed at the
end of the year[1]. This package provides python bindings for `python2`,
however the latest release (also used by other distros) is from 2011[2]
and doesn't support v3. For instance, debian ships `vde2` without
`python2` support since Debian Jessie[3].

KVM and QEMU appear to build fine, also NixOS tests and ISO builds are
still functional.

By running `nix-review` against this change, only `xen` packages failed,
but those were already broken on master[4].

Finally it's also worth mentioning that the closure size of `vde2` drops
from 99.5M to 33.5M without `python2` according to `nix path-info -S -h`.

[1] https://pythonclock.org/
[2] https://github.com/virtualsquare/vde-2/releases/tag/vde-2
    (vde.sourceforge.net redirects to this github page)
[3] https://packages.debian.org/en/jessie/vde2
[4] https://hydra.nixos.org/build/99185451, https://hydra.nixos.org/build/99187262