Revert "tests: use memfd in vhost-user-test"

This reverts commit 7e49f5e8.

This commit seems to break parallel 'make -j4 check';
revert it until we identify the problem.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>