qemu: Use virtio-pci by default for mach-virt guests
    
    virtio-pci is the way forward for aarch64 guests: it's faster
    and less alien to people coming from other architectures.
    Now that guest support is finally getting there (Fedora 24,
    CentOS 7.3, Ubuntu 16.04 and Debian testing all support
    virtio-pci out of the box), we'd like to start using it by
    default instead of virtio-mmio.
    
    Users and applications can already opt-in by explicitly using
    
      <address type='pci'/>
    
    inside the relevant elements, but that's kind of cumbersome and
    requires all users and management applications to adapt, which
    we'd really like to avoid.
    
    What we can do instead is use virtio-mmio only if the guest
    already has at least one virtio-mmio device, and use virtio-pci
    in all other situations.
    
    That means existing virtio-mmio guests will keep using the old
    addressing scheme, and new guests will automatically be created
    using virtio-pci instead. Users can still override the default
    in either direction.
    
    Existing tests such as aarch64-aavmf-virtio-mmio and
    aarch64-virtio-pci-default already cover all possible
    scenarios, so no additions to the test suites are necessary.