drive_get_next() is basically a bad idea.  It returns the "next" block
backend of a certain interface type.  "Next" means bus=0,unit=N, where
subsequent calls count N up from zero, per interface type.

This lets you define unit numbers implicitly by execution order.  If the
order changes, or new calls appear "in the middle", unit numbers change.
ABI break.  Hard to spot in review.

A number of machines connect just one backend with drive_get_next().
Change them to use drive_get() directly.  This makes the (zero) unit
number explicit in the code.

Cc: Beniamino Galvani <b.galvani@gmail.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Subbaraya Sundeep <sundeep.lkml@gmail.com>
Cc: Niek Linnenbank <nieklinnenbank@gmail.com>
Cc: Andrew Baumann <Andrew.Baumann@microsoft.com>
Cc: "Philippe Mathieu-Daudé" <f4bug@amsat.org>
Cc: Jean-Christophe Dubois <jcd@tribudubois.net>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Cc: Bin Meng <bin.meng@windriver.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Artyom Tarasenko <atar4qemu@gmail.com>
Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Cc: Michael Tokarev <mjt@tls.msk.ru>
Cc: Laurent Vivier <laurent@vivier.eu>
Cc: qemu-arm@nongnu.org
Cc: qemu-riscv@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20211117163409.3587705-3-armbru@redhat.com>