vmspawn: close host vsock fd once we passed it to the child

vmspawn: close host vsock fd once we passed it to the child

Without this I qemu simply froze in a weird state for me if I kill it:
it was supposedly a zombie, but we'd get the pidfd POLLIN event for it
only once the fd is closed. Hence let's close it right-away.

(Smells like a kernel issue actually, but too lazy to bother with this).