hw/pci-host/xilinx-pcie: don't make "io" region be RAM

hw/pci-host/xilinx-pcie: don't make "io" region be RAM

Currently we use memory_region_init_rom_nomigrate() to create
the "io" memory region to pass to pci_register_root_bus().
This is a dummy region, because this PCI controller doesn't
support accesses to PCI IO space.

There is no reason for the dummy region to be a RAM region;
it is only used as a place where PCI BARs can be mapped,
and if you could get a PCI card to do a bus master access
to the IO space it should not get acts-like-RAM behaviour.
Use a simple container memory region instead. (We do have
one PCI card model which can do bus master accesses to IO
space -- the LSI53C895A SCSI adaptor.)

This avoids the oddity of having a memory region which is
RAM but where the RAM is not migrated.

Note that the size of the region we use here has no
effect on behaviour.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@mips.com>