Introduce pci_config_read/write_common helpers to prevent passing
accesses down the callback chain that go beyond the config space limits.
Adjust length assertions as they are no longer correct (cutting may
generate valid 3 byte accesses).
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>