pci: allow 0 address for PCI IO/MEM regions

pci: allow 0 address for PCI IO/MEM regions

Some kernels program a 0 address for io regions. PCI 3.0 spec
section 6.2.5.1 doesn't seem to disallow this.

based on patch by Michael Roth <[email protected]>

Add pci_allow_0_addr in MachineClass to conditionally
allow addr 0 for pseries, as this can break other architectures.

This patch allows to hotplug PCI card in pseries machine, as the first
added card BAR0 is always set to 0 address.

This as a temporary hack, waiting to fix PCI memory priorities for more
machine types...

Signed-off-by: Laurent Vivier <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>