qdev: Reject drive property override

qdev_prop_set_drive() screws up when the property already has a
non-null value: it neglects to release the old value.  Both the old
and the new backend become attached to the same device.

Example (taken from iotest 172): -fda ... -drive if=none,... -global
floppy.drive=none0.

Special case: attempting to use the same backend both times fails.
Example (also from iotest 172): -fda ... -global floppy.drive=floppy0.

Yet another example: -device with multiple drive=... (but not
device_add, which silently drops all but the last duplicate property).

Perhaps drive property override could be made to work.  Perhaps it
should.  I can't afford the time to figure this out now.  What I can
do is reject usage that leaves backends in unhealthy states.  For what
it's worth, we've long done the same for netdev properties.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200622094227.1271650-12-armbru@redhat.com>

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