Add NMI Injection Support

control.inject_nmi. diagnostic_sysrq covers soft hangs, but a hard-locked CPU with interrupts disabled ignores sysrq. virsh inject-nmi (paired with in-guest unknown_nmi_panic) converts a hard lockup into a backtrace or a panic→vmcore, feeding the existing postmortem pipeline. Natural sibling of the existing control.* tools, same destructive-op gating question as force_crash.

Start by locating the existing control.* tools, especially diagnostic_sysrq and force_crash, and trace how they feed the postmortem pipeline. Compare the virsh inject-nmi behavior with the existing destructive-operation gating; done means NMI injection is exposed through control.inject_nmi and supports the unknown_nmi_panic backtrace or vmcore flow.

Written by the indexing model from the issue text.