Explicitly specify raw format for drive to qemu_x86_64 to suppress warning

[satnam6502]

Currently we get the warning:

WARNING: Image format was not specified for 'hd.img' 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.

This PR simply adds the raw format specification to the invocation of qemu_x86_64.

    Developer's Certificate of Origin 1.1

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    (b) The contribution is based upon previous work that, to the best
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    Signed-off-by: Satnam Singh <satnam@google.com>

[barrelfish-bot]

Can one of the admins verify this patch?

[antiguru]

Hi Satnam,
thanks for you patch. It seems past versions of qemu had raw disk as default, but that might have changed. I'd be happy to accept this change, but it shouldn't be mixed with Haskell-related changes. These need to be taken care of separately.
Moritz

[satnam6502]

Good point: let me split these apart. On vacation right now, but will try to do this when I have a free moment not scoffing pintxos.