pc: map PCI address space as catchall region for not mapped addresses

pc: map PCI address space as catchall region for not mapped addresses

With a help of negative memory region priority PCI address space
is mapped underneath RAM regions effectively catching every access
to addresses not mapped by any other region.
It simplifies PCI address space mapping into system address space.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
(cherry picked from commit 83d08f2673504a299194dcac1657a13754b5932a)

*prereq for ddaaefb backport

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>