sd: Hide the qdev-but-not-quite thing created by sd_init()

sd: Hide the qdev-but-not-quite thing created by sd_init()

Commit 260bc9d8aa "hw/sd/sd.c: QOMify" QOMified only the device
itself, not its users.  It kept sd_init() around for non-QOMified
users.

More than four years later, three such users remain: omap1 (machines
cheetah, sx1, sx1-v1) and omap2 (machines n800, n810) are not
QOMified, and pl181 (machines integratorcp, realview-eb,
realview-eb-mpcore, realview-pb-a8 realview-pbx-a9, versatileab,
versatilepb, vexpress-a15, vexpress-a9) is not QOMified properly.

The issue I presently have with this: an "sd-card" device should plug
into an "sd-bus" (its DeviceClass member bus_type says so), but
sd_init() leaves it unplugged.  This is normally a bug (I just fixed
some instances), and I'd like to assert proper pluggedness to prevent
regressions.  However, the qdev-but-not-quite thing returned by
sd_init() would fail the assertion.  Meh.

Make sd_init() hide it from QOM/qdev.  Visible in "info qom-tree",
here's the change for cheetah:

     /machine (cheetah-machine)
       [...]
       /unattached (container)
         [...]
         /device[5] (serial-mm)
           /serial (serial)
           /serial[0] (qemu:memory-region)
    -    /device[6] (sd-card)
    -    /device[7] (omap-gpio)
    +    /device[6] (omap-gpio)
         [rest of device[*] renumbered...]

Cc: "Philippe Mathieu-Daudé" <philmd@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200609122339.937862-24-armbru@redhat.com>