fix: harden ARM64 MSHV backend

Configure the GIC, decode MMIO writes, preserve register state, and rebuild the partition during restore.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: cshung <3410332+cshung@users.noreply.github.com>

Author: cshung