qemu/pci: make default_write_config use mask table

Change much of hw/pci to use symbolic constants and a table-driven
design: add a mask table with writable bits set and readonly bits unset.
Detect change by comparing original and new registers.

This makes it easy to support capabilities where read-only/writeable
bit layout differs between devices, depending on capabilities present.

As a result, writing a single byte in BAR registers now works as
it should. Writing to upper limit registers in the bridge
also works as it should. Code is also shorter.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
