Use cross-platform capability in enable_cap example

[rogersnm]

Summary of the PR

Replace the x86-specific KVM_CAP_SPLIT_IRQCHIP capability in the VmFd::enable_cap documentation example with KVM_CAP_HALT_POLL.

KVM_CAP_HALT_POLL is documented as supporting all architectures and is enabled through the VM fd, so the example no longer needs an architecture-specific cfg.

Check support with check_extension_raw before enabling the capability. This keeps the example compatible with older kernels that may not support KVM_CAP_HALT_POLL.

This follows @andreeaflorescu’s suggestion during the review of #382.

Testing

• Ran RUSTDOCFLAGS="-D warnings" cargo doc -p kvm-ioctls --no-deps.
• Compiled the revised doctest with warnings denied.
• Ran Clippy with warnings denied for aarch64, x86_64 and riscv64.
• Ran the example against KVM on aarch64 and x86_64. Both hosts reported KVM_CAP_HALT_POLL as supported and enabled it successfully.
• Ran cargo fmt --all --check.
• Ran git diff --check.

Requirements

• All commits in this PR have Signed-Off-By trailers.
• The changed documentation example is compiled as a doctest.
• No changelog entry is required because this does not change the public API or functionality.
• No new unsafe code is added.