venus: Fix zero-initialized fd causing apps to hang/crash

Some apps such as Gamescope crash under the mere presence of the virtio Vulkan driver without using a device.

This is because virtgpu::fd is zero-initialized upon allocation, which causes fd 0 to be closed in virtgpu_destroy.

Cc: mesa-stable
Fixes: 247232d5 ("venus: add experimental renderers")
Signed-off-by: Joshua Ashton <joshua@froggi.es>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10814>

src/virtio/vulkan/vn_renderer_virtgpu.c

@@ -1579,6 +1579,7 @@ vn_renderer_create_virtgpu(struct vn_instance *instance,
       return VK_ERROR_OUT_OF_HOST_MEMORY;
 
    gpu->instance = instance;
+   gpu->fd = -1;
 
    VkResult result = virtgpu_init(gpu);
    if (result != VK_SUCCESS) {