issue in unplug qemu PCI devices

["Zhai, Edwin" <edwin.zhai@intel.com>]

IOEMU change set e7911109 uses Magic ioport (0x10) protocol for 
negotating with guest PV drivers during startup, and allowing PV drivers 
to disable hardware emulations thus preventing guest from seeing the 
same device through two paths.

But when I tried PV drivers in xen upstream, this unplug logic never 
succeed. PV driver uses following io sequence to interact with 
platform_pci device in qemu:
1. inw 0x10
2. inb 0x12
3. outw(0x12, 0xbeef)
4. outl(0x10,0xdead)

But I only saw 1&2 happened in xen io instruction 
emulation(x86_emulate), while 3&4 seemed to lost so the unplug will 
never happen.

Are you aware of this issue? Or some suggestion for debugging?

BTW, the unplug logic itself has some issues also:
1. Pass-through NICs are also unplugged, although them have different 
path with vnif and emulated NIC.
2. Unplug happens as long as inserting the xen_platform_pci module 
regardless of existence of PVed device in the config file. End user 
likely to unplug all the PCI device by accident.
3. Inserting xen_platform_pci module would unplug all the devices, which 
is not reasonable. E.g. end user only has vbd driver, but all NICs are 
also unplugged.