io-mirror: mirroring framework in qemu block layer

1. Add mirroring attribute to block device:

   usage:

   $ qemu-kvm -cpu host -enable-kvm -m 1024 -smp 2 -drive file=/root/litevirt-livecd/bos.iso -vga qxl -spice port=5902,disable-ticketing -drive file=/root/vm/livecd/data.img,if=none,id=drive0,format=qcow2,cache=directsync,mirroring=tcp://root/a.img -device virtio-blk-pci,scsi=off,drive=drive0,id=disk0 -qmp tcp:localhost:4444,server

2. Allow to work with tcp forwarding backend.