-only-migrate and the two different uses of migration blockers (was: spapr_events: Sure we may ignore migrate_add_blocker() failure?)

[Markus Armbruster]

We appear to use migration blockers in two ways:

(1) Prevent migration for an indefinite time, typically due to use of
some feature that isn't compatible with migration.

(2) Delay migration for a short time.

Option -only-migrate is designed for (1).  It interferes with (2).

Example for (1): device "x-pci-proxy-dev" doesn't support migration.  It
adds a migration blocker on realize, and deletes it on unrealize.  With
-only-migrate, device realize fails.  Works as designed.

Example for (2): spapr_mce_req_event() makes an effort to prevent
migration degrate the reporting of FWNMIs.  It adds a migration blocker
when it receives one, and deletes it when it's done handling it.  This
is a best effort; if migration is already in progress by the time FWNMI
is received, we simply carry on, and that's okay.  However, option
-only-migrate sabotages the best effort entirely.

While this isn't exactly terrible, it may be a weakness in our thinking
and our infrastructure.  I'm bringing it up so the people in charge are
aware :)