error reporting from pre_load etc callbacks

[Peter Maydell <peter.maydell@linaro.org>]

(This was pointed out to me off-list.)

What is the intended API of the pre_load() etc callbacks?
docs/devel/migration.rst doesn't say, and there's no doc comment
in include/migration/vmstate.h. Most actual device implementations
of the hooks seem to return 0 on success and -1 on failure.
But the callers of the hooks like vmstate_load_state() seem to
assume they return negative-errnos: a non-zero return value
is just propagated up to the caller, and can end up for
instance treated as negative-errno in process_incoming_migration_co():
        error_report("load of migration failed: %s", strerror(-ret));
which results in nonsensical error messages to the user like:
 qemu-system-aarch64: load of migration failed: Operation not permitted


More generally, it would be nice if a pre_load/post_load/etc
hook could report an error more precisely than just yes/no
or even than just an errno: in the case of arm CPU loading
we could in theory report more detail like which register the
kernel didn't handle...

thanks
-- PMM