virtio-mmio: improve virtio-mmio get_dev_path alog

virtio-mmio: improve virtio-mmio get_dev_path alog

At the moment the following QEMU command line triggers an assertion
failure On xlnx-versal SOC:
  qemu-system-aarch64 \
      -machine xlnx-versal-virt -nographic -smp 2 -m 128 \
      -fsdev local,id=shareid,path=${HOME}/work,security_model=none \
      -device virtio-9p-device,fsdev=shareid,mount_tag=share \
      -fsdev local,id=shareid1,path=${HOME}/Music,security_model=none \
      -device virtio-9p-device,fsdev=shareid1,mount_tag=share1

  qemu-system-aarch64: ../migration/savevm.c:860:
  vmstate_register_with_alias_id:
  Assertion `!se->compat || se->instance_id == 0' failed.

This problem was fixed on arm virt platform in commit f58b39d2d5b
("virtio-mmio: format transport base address in BusClass.get_dev_path")

It works perfectly on arm virt platform. but there is still there on
xlnx-versal SOC.

The main difference between arm virt and xlnx-versal is they use
different way to create virtio-mmio qdev. on arm virt, it calls
sysbus_create_simple("virtio-mmio", base, pic[irq]); which will call
sysbus_mmio_map internally and assign base address to subsys device
mmio correctly. but xlnx-versal's implements won't do this.

However, xlnx-versal can't switch to sysbus_create_simple() to create
virtio-mmio device. It's because xlnx-versal's cpu use
VersalVirt.soc.fpd.apu.mr as it's memory. which is subregion of
system_memory. sysbus_create_simple will add virtio to system_memory,
which can't be accessed by cpu.

Besides, xlnx-versal can't add sysbus_mmio_map api call too, because
this will add memory region to system_memory, and it can't be added
to VersalVirt.soc.fpd.apu.mr again.

We can solve this by assign correct base address offset on dev_path.

This path was test on aarch64 virt & xlnx-versal platform.

Signed-off-by: schspa <schspa@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>