cmd: Add a -state option to hyperkit runner (and enable vsock)

The state directory is used to store:
- the disk image, if not specified via '-disk'
- the ISO for metadata, if '-data' is used
- the Unix domain socket for virtio sockets
- the PID of the hyperkit process
- a JSON file with the hyperkit configuration

This patch also enables the virtio socket device for the VM.

While at it, also fix .gitignore to ignore kernel images again.

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

.gitignore

@@ -11,6 +11,7 @@ Dockerfile.media
 *.vhdx
 *.efi
 *.qcow2
-*-kernel$
+*-kernel
 *-cmdline
+*-state
 artifacts/*