net/virtio: handle virtio-user setup failure properly

net/virtio: handle virtio-user setup failure properly

This patch fixes virtio_user_dev_setup() error path,
by cleaning all resources it allocates. It introduces
virtio_user_dev_uninit_notify() that cleans all open
FDs. It implies assigning all FDs to -1 at init time.

With these changes done, virtio_user_dev_init_notify()
can be simplified.

Suggested-by: Adrian Moreno <amorenoz@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>