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.
Fixes: 247232d5 ("venus: add experimental renderers")
Signed-off-by: Joshua Ashton email@example.com