Set vmd virtio device fds to -1 on close after fork.

Set vmd virtio device fds to -1 on close after fork.

After the recent vmd(8) commit to clean up file descriptor lifecycles,
virtio disks with multiple file descriptors (QCOW2 images with at
least one base) would fail to initialize when booted with a network
device.

Use the new fd closing routine in the vm process for virtio devices
to close the device fds and set to -1, removing buggy copying and
closing of fds.

Additionally, close the vm/device sync and async channels when
closing a device's fds.

Issue reported by and ok kn@