python: mkvenv: fix messages printed by mkvenv

python: mkvenv: fix messages printed by mkvenv

The new Matcher class does not have a __str__ implementation, and therefore
it prints the debugging representation of the internal object:

  $ ../configure --enable-rust && make qemu-system-arm --enable-download
  python determined to be '/usr/bin/python3'
  python version: Python 3.13.6
  mkvenv: Creating non-isolated virtual environment at 'pyvenv'
  mkvenv: checking for LegacyMatcher('meson>=1.5.0')
  mkvenv: checking for LegacyMatcher('pycotap>=1.1.0')

Add the method to print the nicer

  mkvenv: checking for meson>=1.5.0
  mkvenv: checking for pycotap>=1.1.0

Cc: qemu-stable@nongnu.org
Cc: John Snow <jsnow@redhat.com>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>