Provisioning existing qcow2

[Niraj Nandane]

Hi,

I have downloaded the CentOS qcow2. (CentOS-7-x86_64-GenericCloud.qcow2).

I am trying to provision the qcow2 but getting below (saw when connected to VNC provided by packer):

```
Boot failed: not a bootable disk

Booting from floppy

<then some text here>

No bootable devide

```

packer version : 1.3.4

OS : Mac OS Mojave version 10.14.14

Qemu version : /usr/local/bin/qemu-system-x86_64 --version

QEMU emulator version 4.0.0

Copyright (c) 2003-2019 Fabrice Bellard and the QEMU Project developers

packer json file look like below:

```

{

  "_comment": "Base box builder. We use this output to build all other vms.",

  "builders": [{

    "iso_url": "{{ user `base_vm_path` }}",

    "headless": "{{ user `headless` }}",

    "output_directory": "{{ user `d_output_directory` }}",

    "shutdown_command": "{{ user `shutdown_command` }}",

    "ssh_password": "{{ user `ssh_password` }}",

    "ssh_username": "{{ user `ssh_username` }}",

    "ssh_wait_timeout": "10000s",

    "iso_checksum": "0a60d34921a5e922aeacfeece13bd5ccfb024cb3",

    "iso_checksum_type": "sha1",

    "type": "qemu",

    "format" : "qcow2",

    "vm_name": "{{ user `vm_name` }}",

    "boot_wait" : "20s",

    "cpus" : 4,

    "disk_image" : true,

    "disk_size" : "250000",

    "boot_command": [],

    "accelerator": "none",

    "memory" : "4096",

    "net_device": "virtio-net",

    "disk_interface": "virtio"

  }],

  "post-processors": [],

  "provisioners": []

}

```