Convert an existing disk image to another format using qemu-img

    CLI Example:

    .. code-block:: bash

        salt '*' qemu_img.convert /path/to/original.img /path/to/new.img qcow2

def convert(orig, dest, fmt):
    '''
    Convert an existing disk image to another format using qemu-img

    CLI Example:

    .. code-block:: bash

        salt '*' qemu_img.convert /path/to/original.img /path/to/new.img qcow2
    '''
    cmd = ('qemu-img', 'convert', '-O', fmt, orig, dest)
    ret = __salt__['cmd.run_all'](cmd, python_shell=False)
    if ret['retcode'] == 0:
        return True
    else:
        return ret['stderr']