Fix nil pointer dereference on CreateVM

[yitsushi]

What this PR does / why we need it:

The system does not allow to create a VM with both rootfs and initrd
specified, that means we should be able to create a vm without initrd,
but the code had some strange assumption about that:

   160:         _, err = client.PutGuestBootSource(ctx, &fcmodels.BootSource{
   161:                 KernelImagePath: &cfg.BootSource.KernelImagePage,
   162:                 BootArgs:        *cfg.BootSource.BootArgs,
=> 163:                 InitrdPath:      *cfg.BootSource.InitrdPath,
   164:         })

At the same time, it can happen with BootArgs too as it's not a required
field.

Which issue(s) this PR fixes:

Fixes #175

Special notes for your reviewer:

Checklist:

• squashed commits into logical changes
• includes documentation
• adds unit tests
• adds or updates e2e tests

[codecov-commenter]

Codecov Report

Merging #176 (382368a) into main (94ab549) will decrease coverage by 0.14%.
The diff coverage is 0.00%.

❗ Current head 382368a differs from pull request most recent head 7455a11. Consider uploading reports for the commit 7455a11 to get more accurate results

@@            Coverage Diff             @@
##             main     #176      +/-   ##
==========================================
- Coverage   47.00%   46.85%   -0.15%     
==========================================
  Files          45       45              
  Lines        1951     1957       +6     
==========================================
  Hits          917      917              
- Misses        930      936       +6     
  Partials      104      104              

Impacted Files	Coverage Δ
infrastructure/firecracker/config.go	0.00% <0.00%> (ø)

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[richardcase]

As an alternative, we could create the fcmodels.BootSource before the api call and set the values if not nil. This works though.

[yitsushi]

I like that more, updated.

[yitsushi]

// I tried to remember why I did this and I remember. First I just solved the initrd problem and then realized bootargs can face the same issue, so fixed that too and my brain did not say "hey you conditionally set 2 fields from a struct that has 3 fields"