shpc: standard hot plug controller

shpc: standard hot plug controller

This adds support for SHPC interface, as defined by PCI Standard
Hot-Plug Controller and Subsystem Specification, Rev 1.0
http://www.pcisig.com/specifications/conventional/pci_hot_plug/SHPC_10

Only SHPC intergrated with a PCI-to-PCI bridge is supported,
SHPC integrated with a host bridge would need more work.

All main SHPC features are supported:
- MRL sensor
- Attention button
- Attention indicator
- Power indicator

Wake on hotplug and serr generation are stubbed out but unused
as we don't have interfaces to generate these events ATM.

One issue that isn't completely resolved is that qemu currently
expects an "eject" interface, which SHPC does not provide: it merely
removes the power to device and it's up to the user to remove the device
from slot. This patch works around that by ejecting the device
when power is removed and power LED goes off.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>