Docker machine qemu driver

I needed a non-libvirt qemu driver, so this is it.

Its initial use is going to be for running the Rancher OS tests, but maybe you'll find a use for it too.

from @fventuri

QEMU

Create machines locally using QEMU. This driver requires QEMU to be installed on your host.

$ docker-machine create --driver=qemu qemu-test

Options:

• --qemu-boot2docker-url: The URL of the boot2docker image. Defaults to the latest available version.
• --qemu-disk-size: Size of disk for the host in MB. Default: 20000
• --qemu-memory: Size of memory for the host in MB. Default: 1024
• --qemu-network-bridge: Name of the virtual bridge to be used for networking. Default: virbr0

The --qemu-boot2docker-url flag takes a few different forms. By default, if no value is specified for this flag, Machine will check locally for a boot2docker ISO. If one is found, that will be used as the ISO for the created machine. If one is not found, the latest ISO release available on boot2docker/boot2docker will be downloaded and stored locally for future use. Note that this means you must run docker-machine upgrade deliberately on a machine if you wish to update the "cached" boot2docker ISO.

This is the default behavior (when --qemu-boot2docker-url=""), but the option also supports specifying ISOs by the http:// and file:// protocols. file:// will look at the path specified locally to locate the ISO: for instance, you could specify --qemu-boot2docker-url file://$HOME/Downloads/rc.iso to test out a release candidate ISO that you have downloaded already. You could also just get an ISO straight from the Internet using the http:// form.

Environment variables:

Here comes the list of the supported variables with the corresponding options. If both environment variable and CLI option are provided the CLI option takes the precedence.

Environment variable	CLI option
QEMU_BOOT2DOCKER_URL	--qemu-boot2docker-url