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>

Makefile

@@ -74,4 +74,4 @@ ci-pr:
 
 .PHONY: clean
 clean:
-	rm -rf bin *.log *-kernel *-cmdline *.img *.iso *.gz *.qcow2 *.vhd *.vmx *.vmdk *.tar
+	rm -rf bin *.log *-kernel *-cmdline *-state *.img *.iso *.gz *.qcow2 *.vhd *.vmx *.vmdk *.tar