OpenVMM/microVM umbrella: processor state across snapshots #1182

Objective. A capture saves every guest-visible piece of processor state, or refuses state that it cannot represent; a restore writes all of it; and a restore rejects unsupported state before it changes the partition. Most children concern the KVM backend.

Children.

Order. No child blocks another.

Notes.

• openvmm: KVM snapshot restore loses guest SPEC_CTRL mitigation settings #1141 and openvmm: KVM VP snapshots omit guest-writable MSRs across several CPU features #1160 both add capability-gated optional VP state, and both must decide how older snapshots and readers behave; give them one mechanism and one policy. openvmm: KVM VP snapshots omit guest-writable MSRs across several CPU features #1160 may instead hide some of its CPUID features.
• openvmm: KVM snapshots silently lose pending SMI and SMM execution state #1158, openvmm: KVM snapshot restore collapses two pending NMIs into one #1161, and openvmm: KVM snapshot restore drops pending exceptions and panics on backend-normalized CPU state in debug builds #1169 all change KvmVpStateAccess::{activity, set_activity}; land them one at a time. openvmm: KVM CPU reset retains a pending NMI and interrupt shadow #1159 (CPU reset), which moved to OpenVMM/microVM umbrella: OpenVMM issues outside the microVM and NVX implementation #1186 because a microVM never resets its vCPUs, changes the same functions, and openvmm: KVM snapshot restore collapses two pending NMIs into one #1161 asks to stay separate from its validity-flag fix.
• openvmm: KVM snapshots silently lose pending SMI and SMM execution state #1158, openvmm: unsupported snapshot state panics restore after potentially changing the partition #1168, and openvmm: KVM snapshot restore drops pending exceptions and panics on backend-normalized CPU state in debug builds #1169 all change the generated save_all or restore_all in vmm_core/virt/src/state.rs.