Assigning devices using VFIO allows the guest to have direct access to the
device, whilst filtering accesses to sensitive areas by trapping config
space accesses and mapping DMA with an IOMMU.

This patch adds a new option to lkvm run: --vfio-pci=<BDF>. Before
assigning a device to a VM, some preparation is required. As described in
Linux Documentation/vfio.txt, the device driver needs to be changed to
vfio-pci:

  $ dev=0000:00:00.0

  $ echo $dev > /sys/bus/pci/devices/$dev/driver/unbind
  $ echo vfio-pci > /sys/bus/pci/devices/$dev/driver_override
  $ echo $dev > /sys/bus/pci/drivers_probe

Adding --vfio-pci=$dev to lkvm-run will pass the device to the guest.
Multiple devices can be passed to the guest by adding more --vfio-pci
parameters.

This patch only implements PCI with INTx. MSI-X routing will be added in a
subsequent patch, and at some point we might add support for passing
platform devices to guests.

Reviewed-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>