blockdev: Reject multiple definitions for the same drive

We silently ignore multiple definitions for the same drive:

    $ qemu-system-x86_64 -nodefaults -vnc :1 -S -monitor stdio -drive if=ide,index=1,file=tmp.qcow2 -drive if=ide,index=1,file=nonexistant
    QEMU 0.13.50 monitor - type 'help' for more information
    (qemu) info block
    ide0-hd1: type=hd removable=0 file=tmp.qcow2 backing_file=tmp.img ro=0 drv=qcow2 encrypted=0

With if=none, this can become quite confusing:

    $ qemu-system-x86_64 -nodefaults -vnc :1 -S -monitor stdio -drive if=none,index=1,file=tmp.qcow2,id=eins -drive if=none,index=1,file=nonexistant,id=zwei -device ide-drive,drive=eins -device ide-drive,drive=zwei
    qemu-system-x86_64: -device ide-drive,drive=zwei: Property 'ide-drive.drive' can't find value 'zwei'

The second -device fails, because it refers to drive zwei, which got
silently ignored.

Make multiple drive definitions fail cleanly.

Unfortunately, there's code that relies on multiple drive definitions
being silently ignored: main() merrily adds default drives even when
the user already defined these drives.  Fix that up.

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

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