Avoid warning when using raw format.
Avoid warning when using raw format.

When using the raw image format and attempting to resize it we get the following error message:
```
WARNING: Image format was not specified for 'test.raw' and probing guessed raw.
         Automatically detecting the format is dangerous for raw images, write operations on block 0 will be restricted.
         Specify the 'raw' format explicitly to remove the restrictions.
```
Specifying the format will remove this warning.
ChrisLundquist/packer,bryson/packer,paulmey/packer,pecigonzalo/packer,ChrisLundquist/packer,ChrisLundquist/packer,paulmey/packer,youhong316/packer,dave2/packer,bryson/packer,KohlsTechnology/packer,timsutton/packer,dave2/packer,mitchellh/packer,mkuzmin/packer,hashicorp/packer,timsutton/packer,ricardclau/packer,bryson/packer,bryson/packer,grubernaut/packer,ChrisLundquist/packer,ricardclau/packer,dave2/packer,tas50/packer,timsutton/packer,ricardclau/packer,grubernaut/packer,tas50/packer,tb3088/packer,tas50/packer,mitchellh/packer,paulmey/packer,tb3088/packer,ChrisLundquist/packer,rnaveiras/packer,tas50/packer,rnaveiras/packer,ricardclau/packer,tb3088/packer,paulmey/packer,dave2/packer,hashicorp/packer,mitchellh/packer,dave2/packer,KohlsTechnology/packer,timsutton/packer,youhong316/packer,KohlsTechnology/packer,hashicorp/packer,rnaveiras/packer,pecigonzalo/packer,timsutton/packer,rnaveiras/packer,mkuzmin/packer,tb3088/packer,tas50/packer,mkuzmin/packer,mkuzmin/packer,bryson/packer,hashicorp/packer,grubernaut/packer,ricardclau/packer,youhong316/packer,mkuzmin/packer,ricardclau/packer,KohlsTechnology/packer,paulmey/packer,youhong316/packer,KohlsTechnology/packer,mitchellh/packer,bryson/packer,grubernaut/packer,pecigonzalo/packer,youhong316/packer,ChrisLundquist/packer,timsutton/packer,tb3088/packer,grubernaut/packer,tb3088/packer,mitchellh/packer,KohlsTechnology/packer,tas50/packer,rnaveiras/packer,mkuzmin/packer,youhong316/packer,pecigonzalo/packer,paulmey/packer,rnaveiras/packer,dave2/packer,grubernaut/packer