feat(virtio-block): Add support for VIRTIO_BLK_T_DISCARD request type

[LDagnachew]

(Reupload) Opening this as a Draft PR, as I am still working on fully integrating unmap/fstrim/discard. Right now I'm currently working on the functionality.

Changes

Expose discard feature bit

• Added VIRTIO_BLK_F_DISCARD to the device’s advertised avail_features so guests can negotiate discard support

Extend RequestType enum

• Introduced a new Discard variant and mapped VIRTIO_BLK_T_DISCARD → RequestType::Discard in the request conversion

Update request parsing (Request::parse) in request.rs

• Allowed Discard (alongside Flush) to skip the data descriptor
• Enforced sector‑aligned length and bounds checks for Discard requests

Implement handle_discard backend

• Created handle_discard(offset, len) to call fallocate() on the host file descriptor

Reason

Attempting to resolve Issue #2708

Next Steps

• Checking for VIRTIO_BLK_F_DISCARD support (need suggestions for this, not sure of an efficient way to check for Discard Support within a given VirtIO block).
• Writing Unit & Integration Tests.

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• This functionality cannot be added in rust-vmm.

[LDagnachew]

As far as checking whether the system even supports fstrim/unmap/discard, a trivial way would be to simply check whether fallocate() returns an error, however not entirely sure if there's a better way to do this. @ShadowCurse