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