pci: Add pci-testdev PCI bus test device

Cc: Thomas Huth <thuth@redhat.com>
Cc: Andrew Jones <drjones@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Message-Id: <aa209d81d593909bfaf89bb3a91ff7f33f8441a8.1478512824.git.agordeev@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>