query-block's specification documents response member "type" with
values "hd", "cdrom", "floppy", "unknown".

Its value is unreliable: a block device used as floppy has type
"floppy" if created with if=floppy, but type "hd" if created with
if=none.

That's because with if=none, the type is at best a declaration of
intent: the drive can be connected to any guest device.  Its type is
really the guest device's business.  Reporting it here is wrong.

No known user of QMP uses "type".  It's unlikely that any unknown
users exist, because its value is useless unless you know how the
block device was created.  But then you also know the true value.

Fixing the broken value risks breaking (hypothetical!) clients that
somehow rely on the current behavior.  Not fixing the value risks
breaking (hypothetical!) clients that rely on the value to be
accurate.  Can't entirely avoid hypothetical lossage.  Change the
value to be always "unknown".

This makes "info block" always report "type=unknown".  Pointless.
Change it to not report the type.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>