fw_cfg is needed to get the top of memory which is necessary for
doing PCI allocation and allocating RAM for DMA.

Add a PC version of fw_cfg and enough abstraction to support other
platforms.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Message-id: 1366123521-4330-4-git-send-email-aliguori@us.ibm.com