Allow for optional hardware virtualisation 

I have a requirement to run qemu on a Xen domain with no hardware virtualisation available so I don't want to use kvm or xen. I've also disabled SDL in headless mode and added the net device as done in https://github.com/benh57/packer to get packer qemu working on Oracle Linux 6.

Typical builder json file would look like this and is modelled on a CentOS 6 system.

[[[
{
  "builders":
  [
    {
      "type": "qemu",
      "qemu_binary": "/usr/libexec/qemu-kvm",
      "qemuargs": [
        [ "-no-kvm","" ]
      ],
      "iso_url": "http://kernel.us.oracle.com/osinstall/OracleLinux/OL6/U4/x86_64/OracleLinux-R6-U4-Server-x86_64-dvd.iso",
      "iso_checksum": "4ecb72ade0186afe2f95ffc8273fc146e177e2f9",
      "iso_checksum_type": "sha1",
      "output_directory": "bakery_output_oel64",
      "shutdown_command": "shutdown -P now",
      "disk_size": 10000,
      "format": "raw",
      "headless": true,
      "accelerator": "none",
      "http_directory": "httpdir",
      "http_port_min": 10082,
      "http_port_max": 10089,
      "ssh_host_port_min": 2222,
      "ssh_host_port_max": 2229,
      "ssh_username": "root",
      "ssh_password": "root",
      "ssh_port": 22,
      "ssh_wait_timeout": "90m",
      "vm_name": "oel6.4-vanilla-bakery",
      "net_device": "virtio-net-pci",
      "disk_interface": "virtio",
      "boot_wait": "5s",
      "boot_command":
      [
          "<tab> ks=http://{{ .HTTPIP }}:{{ .HTTPPort }}/kickstart.cfg<enter><wait>"
      ]
    }
  ]
}
]]]

Kickstart file looks like:

[[[
cdrom
lang en_US.UTF-8
keyboard us
network --bootproto=dhcp
rootpw root
firewall --disabled
authconfig --enableshadow --passalgo=sha512
selinux --disabled
timezone UTC
bootloader
 
text
skipx
zerombr
 
clearpart --all --initlabel
part / --size=1 --grow
part swap --recommended
 
reboot

%packages --nobase
at
acpid
cronie-anacron
crontabs
logrotate
mailx
mlocate
openssh-clients
openssh-server
rsync
tmpwatch
which
wget
yum
-biosdevname
-prelink
%end
 
%post
 
# speed up SSH login by avoiding DNS lookup
echo "UseDNS no" >> /etc/ssh/sshd_config
 
# disable IPv6, http://wiki.centos.org/FAQ/CentOS6#head-d47139912868bcb9d754441ecb6a8a10d41781df
echo "# Disable IPv6" >> /etc/sysctl.conf
echo "net.ipv6.conf.all.disable_ipv6 = 1" >> /etc/sysctl.conf
echo "net.ipv6.conf.default.disable_ipv6 = 1" >> /etc/sysctl.conf
sync
%end
]]]