Add packed virtqueue support

[prakash-shekhar]

Summary of the PR

Adds support for packed virtqueues, implementing the VirtIO packed virtqueue format alongside the existing split virtqueue support. Packed virtqueues offer improved performance through better cache locality and reduced memory overhead compared to traditional split virtqueues.

Changes:

1. VringConfigData structure (src/backend.rs):
   - Updated documentation to clearly distinguish between split and packed virtqueue address semantics
   - For packed virtqueues: desc_table_addr becomes the packed descriptor ring, used_ring_addr becomes driver event suppression, avail_ring_addr becomes device event suppression
2. vhost-user protocol support (src/vhost_user/message.rs):
   - Added RING_PACKED feature flag (bit 34, value 0x4_0000_0000)
   - Added VHOST_VRING_F_PACKED vring flag (bit 1, value 0x2)
   - Implemented packed virtqueue inflight I/O structures (DescStatePacked, QueueRegionPacked)
3. vhost-kern support (src/vhost_kern/mod.rs):
   - Extended kernel backend to support packed virtqueue address configuration
   - Added handling of packed layout in vring setup
4. Test coverage:
   - Added test_packed_virtqueue_features() to validate feature flag definitions
   - Added test_packed_vring_addr_validation() to test packed virtqueue address validation/alignment

Requirements

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• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.