runstate_set(): Check for valid transitions

This commit could have been folded with the previous one, however
doing it separately will allow for easy bisect and revert if needed.

Checking and testing all valid transitions wasn't trivial, chances
are this will need broader testing to become more stable.

This is a transition table as suggested by Lluís Vilanova.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
