Add a private_ram flag to VaMapper that enables a mode where the backing
SparseMapping uses committed anonymous memory instead of file-backed
mappings for guest RAM.

In private-RAM mode:
- page_fault() on Windows commits 64KB-aligned chunks via
  SparseMapping::commit() and returns PageFaultAction::Retry, allowing
  the hypervisor to retry the faulting access
- page_fault() on Linux returns PageFaultAction::Fail since the kernel
  handles faults transparently for MAP_ANONYMOUS regions
- alloc_range() eagerly commits a range of pages
- decommit() releases pages back (for future balloon/free-page-reporting)

Add new_private_mapper() to MappingManagerClient that creates a VaMapper
in private-RAM mode, bypassing the mapper cache since private mappers
cannot be shared across partitions.

Add a private_memory option to GuestMemoryBuilder that allocates guest
RAM from committed anonymous virtual memory instead of a shared memory
section.

When private_memory is enabled:
- No shared memory file/section is created (guest_ram is None)
- A private VaMapper is used via new_private_mapper()
- RAM ranges are eagerly committed with alloc_range()
- File-backed add_mapping() calls are skipped for RAM regions
- Memory prefetch is disabled (pages are already committed)

Validation rejects private_memory combined with:
- Legacy memory layout (x86_legacy_support) which requires shared memory
- Pre-existing memory backing (shared_memory_region_base)

The guest_ram field is changed to Option<Mappable> to reflect that
private memory mode does not produce a shareable backing object.

Add the --private-memory CLI flag to OpenVMM and wire it through the
configuration pipeline:

- Add private_memory field to MemoryConfig in openvmm_defs
- Add --private-memory boolean flag to CLI argument parser
- Wire the flag from CLI args to MemoryConfig in openvmm_entry
- Pass private_memory to GuestMemoryBuilder in dispatch.rs
- Default private_memory to false in ttrpc and petri config construction

Add unit tests covering the new private memory functionality:

sparse_mmap (3 tests):
- test_decommit_zeros_pages: verify decommit returns pages to zero
- test_commit_after_decommit: verify commit restores access after decommit
- test_commit_idempotent: verify commit on already-committed pages is safe

membacking (4 tests):
- test_private_ram_alloc_write_read: basic alloc/write/read cycle
- test_private_ram_decommit_zeros: verify decommit zeros private pages
- test_private_ram_recommit_after_decommit: verify recommit after decommit
- test_private_ram_commit_idempotent: verify double-commit is safe

All tests use direct SparseMapping operations to avoid async infrastructure
complexity, exercising the actual platform commit/decommit code paths.

- shared_memory_backing() now returns Option<SharedMemoryBacking> instead
  of panicking in private memory mode
- Restart path returns a proper error when private memory is active
  instead of hitting an expect() abort
- VaMapper private-RAM fault paths use GuestMemoryErrorKind::Other
  instead of OutOfRange, since commit failures may be resource-related
  rather than address-range issues

Use std::io::Error::other() instead of std::io::Error::new(ErrorKind::Other, ...)
as required by the io_other_error clippy lint.