vm_phys_early_alloc(): Add diagnostics and comments

[olce]

Summary

Panic after each search for biggest ranges if no range was found at all.

If there is not enough space for the allocation, print relevant information
when panicking.

Check (under invariants) that 'alloc_size' is not 0, as this case is not
currently supported and cannot be triggered by current callers.

Add a few comments.