iotests/172: Cover -global floppy.drive=...

Use of -global to set a default backend for non-singleton devices is a
bad idea.  But as long as we permit it, we better test it.

Test output demonstrates we screw up when -global floppy clashes with
-fda or with -device floppy: according to "info qtree", only the
latter backend is attached, but according to "info block", both are.
Here's the clash with -device:

    Testing: -drive if=none,file=TEST_DIR/t.qcow2 -drive if=none,file=TEST_DIR/t.qcow2.2 -global floppy.drive=none0 -device floppy,drive=none1,unit=0

              dev: isa-fdc, id ""
                [...]
                driveA = ""
                driveB = ""
                [...]
                bus: floppy-bus.0
                  type floppy-bus
                  dev: floppy, id ""
                    unit = 0 (0x0)
--->                drive = "none1"
    [...]
    none0 (NODE_NAME): TEST_DIR/t.qcow2 (qcow2)
--->    Attached to:      /machine/peripheral-anon/device[0]
        Cache mode:       writeback

    none1 (NODE_NAME): TEST_DIR/t.qcow2.2 (qcow2)
--->    Attached to:      /machine/peripheral-anon/device[0]
        Removable device: not locked, tray closed
        Cache mode:       writeback

/machine/peripheral-anon/device[0] is the floppy created with -device.

Test output further demonstrates the "Drive 'FOO' is already in use
because it has been automatically connected to another device" error
message can be misleading.  With '-fda "" -global
floppy.drive=floppy0', it's in use because -global reuses -fda's
backend.  There is no other device involved.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200622094227.1271650-4-armbru@redhat.com>

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