vhost-net: compile it on all targets that have virtio-net.

vhost-net: compile it on all targets that have virtio-net.

This shows a preexisting bug: if a KVM target did not have virtio-net enabled,
it would fail with undefined symbols when vhost was enabled.  This must now
be fixed, lest targets that have no virtio-net fail to compile.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1543851204-41186-5-git-send-email-pbonzini@redhat.com>
Message-Id: <1550165756-21617-6-git-send-email-pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>