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.

The aspeed machines connects backends with drive_get_next() in several
counting loops, one of them in a helper function, and a conditional.
Change it to use drive_get() directly.  This makes the unit numbers
explicit in the code.

Cc: "Cédric Le Goater" <clg@kaod.org>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Andrew Jeffery <andrew@aj.id.au>
Cc: Joel Stanley <joel@jms.id.au>
Cc: qemu-arm@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20211117163409.3587705-13-armbru@redhat.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>