$qemu-imgcreate-fqcow2image.img20G
This is what I did to set up bridged networking for my KVM guests:
# vim /etc/network/interfaces
And set my wired interface to manual and define a bridge interface
br0 which my KVM guests will use.
iface eth0 inet manual
post-up ifup br0
iface br0 inet dhcp
bridge_ports eth0
Starting the KVM guest by booting from its CDROM using the Debian stable ISO as the CD:
#kvm-netnic,macaddr=00:00:00:00:00:04\
-nettap\
-cdrom/home/torstein/tmp/debian-8.1.0-amd64-netinst.iso\
-bootd
-m1024\
/home/torstein/virtual/wind.img