migration: Add more error handling to analyze-migration.py

migration: Add more error handling to analyze-migration.py

The analyze-migration script was seen failing in s390x in misterious
ways. It seems we're reaching the VMSDFieldStruct constructor without
any fields, which would indicate an empty .subsection entry, a
VMSTATE_STRUCT with no fields or a vmsd with no fields. We don't have
any of those, at least not without the unmigratable flag set, so this
should never happen.

Add some debug statements so that we can see what's going on the next
time the issue happens.

Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20250109185249.23952-2-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
(cherry picked from commit 86bee9e0c761a3d0e67c43b44001fd752f894cb0)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>