vhost-user: disable the *broken* subprocess tests

tests/vhost-user-test keeps failing on build-system since Aug 15:

  ERROR:tests/vhost-user-test.c:835:test_flags_mismatch: child process (/i386/vhost-user/flags-mismatch/subprocess [4836]) failed unexpectedly
...
  ERROR:tests/vhost-user-test.c:807:test_connect_fail: child process (/x86_64/vhost-user/connect-fail/subprocess [58910]) failed unexpectedly

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Suggested-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20170905180602.28698-1-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
