Virtio add feature support  for "any layout" and "indirect descriptors"

This patchseries adds teh following changes to virtio device imlementation:
 - 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 

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)