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 versatile and vexpress machines ("versatileab", "versatilepb",
"vexpress-a9", "vexpress-a15") connect just one or two backends of a
type with drive_get_next().  Change them to use drive_get() directly.
This makes the unit numbers explicit in the code.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-arm@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20211117163409.3587705-5-armbru@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>