core: Rework PCI config space access handling

core: Rework PCI config space access handling

Move more logic into generic code by extending the write handler to
pci_cfg_write_moderate and introducing pci_cfg_read_moderate. These
handlers are responsible for any config space access, including to
unowned or non-existent devices. They can reject the access, return an
emulated value on read or a real value to be written to hardware, or
they instruct the caller to perform the access directly.

We already pass a reference to the issuing cell to the access handlers.
It stays unused for now but will be needed by succeeding changes. So
add it now to avoid changing API and callers once again later on.

This commit lays the foundation for capability access moderation and,
specifically, MSI/MSI-X emulation.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>