nvx-rs: print zero CPUID fields in nvx-time-probe clock as C's %#x does #590

Summary

nvx-time-probe clock prints the hypervisor's CPUID fields with Rust's {:#x}, which prints zero as 0x0, where the C program's %#x printed 0. The summary line's hv_max_leaf, hv_features_eax, and hv_features_edx are all zero when the hypervisor bit is clear, and its feature fields are zero under a hypervisor whose leaves end before 0x40000003. The cpuid.0x40000000 line's eax.max_leaf is likewise zero for a hypervisor whose leaf reads zero. Each such valid run differs from the C program's byte-exact output.

Found in the #550 review. The port has differed since its first revision, and #550 leaves the fix to this issue. #589 is the same difference in the MSR values.

Proposal

Print these fields with the helper for C's %#x that #589 proposes, which prints zero without the prefix, as alternate_hex_width_10 already does for %#010x.

Tasks

• Print zero as 0 in the summary line's hv_max_leaf, hv_features_eax, and hv_features_edx, and in the eax.max_leaf that print_cpuid prints, keeping 0x before nonzero values.
• Add a regression for the zero-valued summary of a CPU without the hypervisor bit, with the C program's output as the expected result.