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).

src/vmspawn/vmspawn.c

@@ -696,6 +696,9 @@ static int run_virtual_machine(void) {
                 _exit(EXIT_FAILURE);
         }
 
+        /* Close the vsock fd we passed to qemu in the parent. We don't need it anymore. */
+        child_vsock_fd = safe_close(child_vsock_fd);
+
         int exit_status = INT_MAX;
         if (use_vsock) {
                 r = setup_notify_parent(event, vsock_fd, &exit_status, &notify_event_source);