[Libguestfs] [PATCH] aarch64: Enable virtio-pci, replacing virtio-mmio.

This patch causes aarch64 to use virtio-pci instead of virtio-mmio. Virtio-pci is considerably faster than virtio-mmio, it's more like how other architectures work, and it supports hotplugging (although it's not likely we'd use the latter feature). I'm not necessarily suggesting that we apply this. Laine (CC'd) has some further patches to libvirt lined up which AIUI would make it simpler to do this. I have given this a little bit of testing and it works fine for simple operations on aarch64. I'm currently running the in-depth libguestfs test suite. Rich.