ps2: migrate ledstate

ps2: migrate ledstate

Make the ps2 device track its ledstate so that we can migrate it.
Otherwise it gets lost across migration, and spice-server gets
confused about the actual keyboard state and sends bogus
caps/scroll/num key events. This fixes RH bug #729294

We only need to migrate the state when it is different of the default
one (0).

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>