[PCI] Allow per-device configuration for fine-grained control over PCI

[PCI] Allow per-device configuration for fine-grained control over PCI
configuration space writes, with a goal that was also previously
described by Ryan:

"Permissive mode should only be used as a fall back for unknown
devices.
I think the correct solution for dealing with these device-specific
configuration space registers is to identify them and add the
device-specific fields to the overlay. This patch adds a special
configuration space handler for network cards based on the tg3 linux
network device driver. This handler should allow for reads/writes to
all of the configuration space registers that the tg3 driver requires."

This patch attempts to address concerns with Ryan's original
submission by moving policy from the dom0 kernel into dom0 user-space.
As new quirky devices emerge they can be incorporated into the user-space
policy.  An added benefit is that changes to the policy are effective
for domains created after the changes are written (no need rebuild the
hypervisor or restart xend).

Signed-off-by: Chris Bookholt <hap10@tycho.ncsc.mil>