vm.rs: Replace hardcoded arch::RAM_START in BootInfo

[n0toose]

There was an idea to initialize the mmap using different guest_address parameters.

Despite how most of the code is already "future-proofed", with a clear intention of heading towards that direction and vm.rs using self.mem.guest_address everywhere (despite that value currently being equal to zero), this change modifies a small exception to that rule.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 68.06%. Comparing base (efc3563) to head (fa93dd3).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #712 +/- ## ======================================= Coverage 68.06% 68.06% ======================================= Files 20 20 Lines 2311 2311 ======================================= Hits 1573 1573 Misses 738 738 ```

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