hw/dma/i82374: Avoid double creation of the 82374 controller

hw/dma/i82374: Avoid double creation of the 82374 controller

QEMU fails when used with the following command line:

    ./ppc64-softmmu/qemu-system-ppc64 -S -machine 40p -device i82374
    qemu-system-ppc64: hw/isa/isa-bus.c:110: isa_bus_dma: Assertion `!bus->dma[0] && !bus->dma[1]' failed.

The 40p machine type already creates the device i82374. If specified in the
command line, it will try to create it again, hence generating the error. The
function isa_bus_dma() isn't supposed to be called twice for the same bus.
Check the bus doesn't already have a DMA controller registered before creating
the device.

Fixes: https://bugs.launchpad.net/qemu/+bug/1721224
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180326153441.32641-2-f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>