fix: Preserve kernel argument ordering

[DZDomi]

Currently the SDK uses a map for storing the kernel arguments. This map will then be serialised back into a string before it is passed to the firecracker process. Since the iteration of a map in go is random and not deterministic the kernel params will be given in any order, which can result in a failure in the following case:

When passing init arguments to the kernel with the -- flag the SDK will save it correctly in the map, but after turning it back into a string the order of the arguments is not correct.

As an example with the following arguments:

reboot=k panic=-1 quiet loglevel=1 console=ttyS0 -- --argument-for-init ENV_FOR_INIT=somevalue

Before the fix this could result in:

reboot=k -- panic=-1  ENV_FOR_INIT=somevalue quiet loglevel=1 console=ttyS0 --argument-for-init

Which would not pass these arguments into the init or even worse prevents the VM from starting.

After the fix the order of the arguments is always preserved and will be given like this to the kernel:

reboot=k panic=-1 quiet loglevel=1 console=ttyS0 -- --argument-for-init ENV_FOR_INIT=somevalue

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.