vhost-user-test: make features mask an init_virtio_dev() argument

The goal is to generalize the use of [un]init_virtio_dev() to
all tests, which does not necessarily expose the same features
set.

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