pci: edu: introduce pci-edu helpers

QEMU edu device is a pci device that is originally written for
educational purpose, however it also suits for IOMMU unit test. Adding
helpers for this specific device to implement the device logic.

The device supports lots of functions, here only DMA operation is
supported.

The spec of the device can be found at:

  https://github.com/qemu/qemu/blob/master/docs/specs/edu.txt



Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>