The qemu part of this patch can be found in
https://lists.gnu.org/archive/html/qemu-devel/2018-09/msg01988.html
pxb-pcie device in qemu uses only one pci domain(0) so far, which means
there are at most 256 busses. However, pcie topology requires one bus per
device, which will use up the busses soo
To support multiple pci domains of pxb-pcie device in qemu, we need to setup
mcfg range in seabios. We use [0x8000, 0xb000) to hold new domain mcfg
table for now, and we need to retrieve the desired mcfg size of each pxb-pcie
from a hidden bar because they may not need the whole 256 busses,
