Virtio features

This is a merge of the following branches/features:

 - "old" MQ branch - implementing ANY_LAYOUT, VQ/MQ and INDIRECT_DESCRIPTORS
 - Antonio's v-mq branch - where he improves the MQ branch performance with some luajit trickery
 - the "new" MRG_RXBUF support

The current performance of this branch is (10e7 64byte packets):
On 0000:05:00.0 got 3.699
On 0000:82:00.0 got 3.547
On 0000:84:00.0 got 3.517
On 0000:88:00.0 got 3.616
Rate(Mpps):     14.379

Iperf with MTU 9100:

iperf -c 10.0.0.1

Client connecting to 10.0.0.1, TCP port 5001
TCP window size:  325 KByte (default)

[  3] local 10.0.0.2 port 50996 connected with 10.0.0.1 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec  7.12 GBytes  6.12 Gbits/sec

Jumbo frame support:
ping -c1 -s9000 10.0.0.1      
PING 10.0.0.1 (10.0.0.1) 9000(9028) bytes of data.
9008 bytes from 10.0.0.1: icmp_seq=1 ttl=64 time=0.756 ms

--- 10.0.0.1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.756/0.756/0.756/0.000 ms