arm: Add PCIe host bridge in virt machine

Now that we have a working "generic" PCIe host bridge driver, we can plug
it into ARM's virt machine to always have PCIe available to normal ARM VMs.

I've successfully managed to expose a Bochs VGA device, XHCI and an e1000
into an AArch64 VM with this and they all lived happily ever after.

Signed-off-by: Alexander Graf <agraf@suse.de>
Tested-by: Claudio Fontana <claudio.fontana@huawei.com>
[PMM: Squashed in fix for off-by-one error in bus-range DT property
 from Laszlo Ersek <lersek@redhat.com>]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>