sdhci: Make device "sdhci-pci" unavailable with -device

Device models aren't supposed to go on fishing expeditions for
backends.  They should expose suitable properties for the user to set.
For onboard devices, board code sets them.

"sdhci-pci" picks up its block backend in its realize() method with
drive_get_next() instead.  Already marked FIXME.  See the commit that
added the FIXME for a more detailed explanation of what's wrong.

We can't fix this in time for the release, but since the device is new
in 2.3, we can set cannot_instantiate_with_device_add_yet to disable
it before this mistake becomes ABI, and we have to support command
lines like

    $ qemu -drive if=sd -drive if=sd,file=sd.img -device sdhci-pci -device sdhci-pci

forever.

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