nvx-rs: print zero MSR values in nvx-time-probe clock --msr as C's %#x does #589

Summary

nvx-time-probe clock --msr prints each MSR's value with Rust's {:#x}, which prints zero as 0x0. The C program printed it with %#" PRIx64, whose alternate form prefixes 0x to nonzero values only, so a zero MSR prints = 0x0 (0) where C printed = 0 (0). CPU 0's HV_X64_MSR_VP_INDEX, which the probe reads, is always zero, so every clock --msr run that can read the MSRs 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. #590 is the same difference in the CPUID fields.

Proposal

Add a helper for C's %#x, which prints zero without the prefix, as alternate_hex_width_10 in nvx-rs/src/guest/time_probe/clock.rs already does for %#010x, and print the MSR values with it. #590 can use the same helper.

Tasks

• Print zero MSR values as 0 in print_msrs, keeping 0x before nonzero values.
• Add a regression that formats zero and nonzero MSR values, with the C program's output as the expected result.