hypervisor, vmm: Fix aarch64 SVE register save/restore

When SVE is enabled, KVM replaces the FPSIMD V-registers with wider
SVE Z-registers. Attempting to access the old FPSIMD offsets returns
EINVAL.

Fix by classifying each register from KVM_GET_REG_LIST as core, system,
or extended. Extended registers (currently SVE only) are saved as
generic `ExtendedReg` entries split into `pre_finalize_regs` (registers
like SVE VLS that must be written before `vcpu_finalize`) and
`extended_regs`. FPSIMD registers are only accessed when SVE is absent.
Unrecognized register families error immediately so future extensions
like SME fail clearly rather than silently losing state.

The snapshot is deserialized before vCPU init to make pre-finalize
register state available for the init -> VLS -> finalize ordering
required by KVM.

Signed-off-by: Ruben Hakobyan <hruben@meta.com>

hypervisor/src/arch/aarch64/mod.rs

@@ -4,3 +4,11 @@
 
 pub mod gic;
 pub mod regs;
+
+use serde::{Deserialize, Serialize};
+
+#[derive(Clone, Serialize, Deserialize)]
+pub struct ExtendedReg {
+    pub id: u64,
+    pub data: Vec<u8>,
+}