qemu/pci: check constant registers on load

Add "cmask" table of constant register masks: if a bit is not writeable
and is set in cmask table, this bit is checked on load.  An attempt to
load an image that would change such a register causes load to fail.
Use this table to make sure that load does not modify registers that
guest can not change (directly or indirectly).

Note: we can't just assume that read-only registers never change,
because the guest could change a register indirectly.

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