hw/pci-host/versatile.c: Update autodetect to detect newer kernels

hw/pci-host/versatile.c: Update autodetect to detect newer kernels

Newer versatilepb kernels still don't get the IRQ mapping right
for the PCI controller, but they get it differently wrong (they add
a fixed +64 offset to everything they write to PCI_INTERRUPT_LINE).
Update the autodetection to handle these too, and include a more
detailed comment on the various different behaviours that might
be present.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1368545616-22344-3-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>