tests: re-enable vhost-user-test

Commit 7fe34ca9c2e actually disabled vhost-user-test altogether,
since CONFIG_VHOST_NET is a per-target config variable.

tests/vhost-user-test is already x86/x64 softmmu specific test, in order
to enable it correctly, kvm & vhost-net are also conditions. To check
that, set CONFIG_VHOST_NET_TEST_$target when kvm is also enabled.

Since "check-qtest-x86_64-y = $(check-qtest-i386-y)", avoid duplication
when both x86 & x64 are enabled.

Other targets than x86 aren't enabled yet, and is intentionally left as
a future improvement, since I can't easily test those.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
