Currently, upon BLOCK_JOB_* event, disk->mirrorState is not updated
each time. The callback code handling the events checks if a blockjob
was started via our public APIs prior to setting the mirrorState.
However, some block jobs may be started internally (e.g. during
storage migration), in which case we don't bother with setting
disk->mirror (there's nothing we can set it to anyway), or other
fields. But it will come handy if we update the mirrorState in these
cases too. The event wasn't delivered just for fun - we've started the
job after all.

So, in this commit, the mirrorState is set to whatever job status
we've obtained. Of course, there are some actions on some statuses
that we want to perform. But instead of if {} else if {} else {} ...
enumeration, let's move to switch().
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>