virttest.qemu_devices: Support for PCI RC/bridge/switch

Hi guys,

the PCI switch required quire a change of qemu_devices. Anyway it should allow us to define automated behavior of various kinds for other bus-related devices. Please read the actual commit logs for details.

I'll write a test, which will use those setups. Currently you can test it manually by modifying `qemu/cfg/tests.cfg`:
<pre>
pci_bus = 'pci_bridge1'
pci_controllers = "root.1 pci_bridge1"
pci_bus_root.1 = 'pci.0'
type_root.1 = 'ioh3420'
pci_bus_pci_bridge1 = 'root.1'
type_pci_bridge1 = 'pci-bridge'
</pre>

or you can add some setups to `qemu/cfg/guest-hw.cfg`:
<pre>
variants:
    - @default_pci:
    - root_controller:
        pci_bus = 'root.1'
        pci_controllers = "root.1"
        pci_bus_root.1 = 'pci.0'
        type_root.1 = 'ioh3420'
    - bridge:
        # PCI2PCI
        pci_bus = 'pci_bridge1'
        pci_controllers = "pci_bridge1"
        pci_bus_pci_bridge1 = 'pci.0'
        type_pci_bridge1 = 'pci-bridge'
        q35:
            # PCIE2PCI
            type_pci_bridge1 = 'i82801b11-bridge'
    - switch:
        pci_bus = 'pci_switch1'
        pci_controllers = "pci_switch1"
        pci_bus_pci_switch1 = 'pci.0'
        type_pci_switch1 = 'x3130'
</pre>

Most of the tests should work apart from the ones relying on 32 devices (pci-bridges supports only 19 devices). Please keep in mind that PCI RC/bridge/switch is kind of new to qemu and you'll see lot's of failures while trying it. I'm using upstream ~qemu-1.6.50 and only single PCI bridge works fine. Root controller is sometimes usable, but not for all devices.

Now I'll focus on some tests which would boot and verify various setups.

Kind regards,
Lukáš