qemu: fix qemuDomainSupportsPCI() for ARM machines of "virt" machinetype
    
    Commit e8d5517 updated the domain post-parse to automatically add
    pcie-root et al for certain ARM "virt" machinetypes, but didn't update
    the function qemuDomainSupportsPCI() which is called later on when we
    are auto-assigning PCI addresses and default settings for the PCI
    controller <model> and <target> attributes. The result was that PCI
    addresses weren't assigned, and the controllers didn't have their
    attribute default values set, leading to an error when the domain was
    started, e.g.:
    
      internal error: autogenerated dmi-to-pci-bridge options not set
    
    This patch adds the same check made in the earlier patch to
    qemuDomainSupportsPCI(), so that PCI address auto-assignment and
    target/model default values will be set.