pci: Split pcie_host_mmcfg_map()

The mmcfg space is a memory region that allows access to PCI config space
in the PCIe world. To maintain abstraction layers, I would like to expose
the mmcfg space as a sysbus mmio region rather than have it mapped straight
into the system's memory address space though.

So this patch splits the initialization of the mmcfg space from the actual
mapping, allowing us to only have an mmfg memory region without the map.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Claudio Fontana <claudio.fontana@huawei.com>
