PCI: hv: Add paravirtual PCI front-end for Microsoft Hyper-V VMs

Add a new driver which exposes a root PCI bus whenever a PCI Express device
is passed through to a guest VM under Hyper-V.  The device can be single-
or multi-function.  The interrupts for the devices are managed by an IRQ
domain, implemented within the driver.

[bhelgaas: fold in race condition fix (http://lkml.kernel.org/r/1456340196-13717-1-git-send-email-jakeo@microsoft.com)]
Signed-off-by: Jake Oshins <jakeo@microsoft.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>