fdc: Throw an assertion on misconfigured fd_formats table

pick_geometry is a convoluted function that makes it difficult to tell
at a glance what QEMU's current behavior for choosing a floppy drive
type is when it can't quite identify the diskette.

The code iterates over all entries in the candidate geometry table
("fd_formats") and if our specific drive type matches a row in the table,
then either "match" is set to that entry (an exact match) and the loop
exits, or "first_match" will be non-negative (the first such entry that
shares the same drive type), and the loop continues. If our specific
drive type is NONE, then all drive types in the candidate geometry table
are considered. After iteration, if "match" was not set, we fall back to
"first match".

This means that either "match" was set, or we exited the loop without an
exact match, in which case:

- If drive type is NONE, the default is truly fd_formats[0], a 1.44MB
  type, because "first_match" will always get set to the first item.

- If drive type is not NONE, pick_geometry's iteration was fussier and
  only looked at rows that matched our drive type. However, since all
  possible drive types are represented in the table, we still know that
  "first match" was set.

- If drive type is not NONE and the fd_formats table lists no options for
  our drive type, we choose fd_formats[1], an incomprehensibly bizarre
  choice that can never happen anyway.

Correct this: If first_match is -1, it can ONLY mean we didn't edit our
fd_formats table correctly. Throw an assertion instead.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1453495865-9649-6-git-send-email-jsnow@redhat.com

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