Virtio add feature support for "any layout", "indirect descriptors" and "multi queue"



This patchseries adds the following changes to virtio device implementation:

    prints negotiated feature flags during the negotiation phase
    implements VirtioVirtq, a common class for virt queue operations
    adds support for "any layout" feature
    adds support for indirect descriptors
    adds support for muti queue

Performance:

    the refactoring to VirtioVirtq slows down the code to 2Mpps with loadgen DPDK benchmark
    adding the indirect descriptors betters the netperf performance on an unpatched QEMU ( 2Gbps vs 4Gbps)