x86-64/MMCFG: correct base address computation for regions not starting at bus 0

x86-64/MMCFG: correct base address computation for regions not starting at bus 0

As per the specification, the base address reported by ACPI is the one
that would be used if the region started at bus 0. Hence the
start_bus_number offset needs to be added not only to the virtual
address, but also the physical one when establishing the mapping, and
it then needs to be subtracted when obtaining the virtual address for
doing accesses.

Signed-off-by: Jan Beulich <jbeulich@novell.com>