hw/dma.c: Fix conversion of ioport_register* to MemoryRegion

hw/dma.c: Fix conversion of ioport_register* to MemoryRegion

The commit 582299336879504353e60c7937fbc70fea93f3da introduced a 1-shift for
some offset in DMA emulation.

Before the previous commit, which converted ioport_register_* to
MemoryRegion, the DMA controller registered 8 ioports with the following
formula:
base + ((8 + i) << d->shift) where 0 <= i < 8
When an IO occured within a Memory Region, DMA callback receives an
offset relative to the start address. Here the start address is:
base + (8 << d->shift).
The offset should be: (i << d->shift). After the shift is reverted, the
offsets are 0..7 not 1..8.

Fixes LP#1089996.

Reported-by: Andreas Gustafsson <[email protected]>
Signed-off-by: Julien Grall <[email protected]>
Tested-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Andreas Färber <[email protected]>