Support optional isolated venv for Avocado bootstrap

[bssrikanth]

Keep the existing system-wide pip path as default. Add --use-venv / AVOCADO_USE_VENV for a clean virtualenv install that avoids distro package conflicts.

[bssrikanth]

@PraveenPenguin @narasimhan-v could you please review. I have tested this patch against below scenarios:

Venv Patch — Test Scenarios Validated

Result: All Pass

1. Baseline without venv — Run guest suite without --use-venv.
2. Fresh venv bootstrap — Bootstrap with --bootstrap --use-venv --enable-kvm.
3. Guest run in venv mode — Run guest suite with --use-venv.
4. Venv clean — Run --clean --use-venv.
5. Venv rebuild after clean — Re-bootstrap with --use-venv after venv clean.
6. Guest run with existing image — Run with --use-venv --no-download when guest image is present.
7. Guest re-download when image missing — Run with --use-venv without --no-download.
8. Switch from venv to system mode — Run guest suite without --use-venv after venv usage.
9. Switch from system back to venv — Run guest suite with --use-venv after system bootstrap.
10. Recovery after plain clean — Run --clean, then --run-suite --use-venv.
11. Env var bootstrap — Bootstrap with AVOCADO_USE_VENV=1 (no CLI flag).
12. Env var run — Run guest suite with AVOCADO_USE_VENV=1.
13. Orphan venv survives plain clean — After --clean without --use-venv, .venv remains on disk.
14. Run after orphan venv + deep clean — After plain --clean, run with --use-venv.
15. System and venv installs coexist — Bootstrap system-wide, then bootstrap with --use-venv.
16. Venv takes precedence over system — With both installed, run with --use-venv.
17. Bootstrap without guest download — --bootstrap --use-venv --no-download when guest image exists.
18. Run after no-download bootstrap — Run guest suite with --no-download --use-venv.
19. Host-only venv bootstrap — --bootstrap --use-venv without --enable-kvm.
20. Host-only venv run — --run-suite host_sanity --use-venv.

[PraveenPenguin]

LGTM