virttest.asset: Make relative links instead of symbolic ones

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
test-providers.d: Update README with instructions

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
virttest.asset: Consolidate sha1 and tag in ref

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
virttest.bootstrap: Remove unused import

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>