vfkit: More robust Kill()

We know that setting the state to `HardStop` typically fails:

    I0309 19:19:42.378591   21795 out.go:177] 🔥  Deleting "minikube" in vfkit ...
    W0309 19:19:42.397472   21795 delete.go:106] remove failed, will retry: kill: Post "http://_/vm/state": EOF

This may lead to unnecessary retries and delays. Fix by falling back to
sending a SIGKILL signal.

Example delete flow when setting vfkit state fails:

    I0309 20:07:41.688259   25540 out.go:177] 🔥  Deleting "minikube" in vfkit ...
    I0309 20:07:41.712017   25540 main.go:141] libmachine: Failed to set vfkit state to 'HardStop': Post "http://_/vm/state": EOF
    I0309 20:07:41.712082   25540 main.go:141] libmachine: Sending signal "killed" to pid 25482

Author: nirs

pkg/drivers/vfkit/vfkit.go

@@ -394,7 +394,9 @@ func (d *Driver) extractKernel(isoPath string) error {
 
 func (d *Driver) Kill() error {
 	if err := d.SetVFKitState("HardStop"); err != nil {
-		return err
+		// Typically fails with EOF.
+		log.Debugf("Failed to set vfkit state to 'HardStop': %s", err)
+		return signalPidfile(d.pidfilePath(), syscall.SIGKILL)
 	}
 	return nil
 }