[process] Hold reference to process during call to step()

[process] Hold reference to process during call to step()

It is conceivable that the process may terminate during the execution
of step().  If nothing else holds a reference to the containing
object, this would cause the object to be freed prior to returning
from step().

Add a ref_get()/ref_put() around the call to ->step() to prevent this
from happening.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>