net_tap: change TapHandle from device name to pre-opened fd

[jstarks]

TapHandle in net_backend_resources carried a device name string and the resolver opened the TAP device. Change it to carry a pre-opened OwnedFd so the opener (openvmm_entry or a test harness) controls namespace and permission details. This resolves a long-standing TODO and enables fd passing from Kata and similar environments.

Also tighten both TapHandle and net_tap from cfg(unix) to cfg(target_os = "linux") since TAP uses Linux-specific /dev/net/tun ioctls.

[Copilot]

Pull request overview

Moves TAP device opening out of the net_tap resolver and into the caller by changing TapHandle (in net_backend_resources) to carry a pre-opened OwnedFd, enabling more control over namespaces/permissions and supporting fd passing scenarios. It also tightens TAP-related code from cfg(unix) to Linux-only since TAP relies on Linux /dev/net/tun ioctls.

Changes:

• Change TapHandle from device name (String) to a pre-opened OwnedFd, and update the TAP resolver to use it directly.
• Move TAP opening to openvmm_entry (CLI and ttrpc/grpc paths) so the entrypoint controls how the fd is obtained.
• Restrict net_tap and TAP resources to target_os = "linux" and update dependencies accordingly.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
vm/devices/net/net_tap/src/resolver.rs	Stop opening TAP by name; construct Tap from the fd in TapHandle.
vm/devices/net/net_tap/src/lib.rs	Make the crate Linux-only (cfg(target_os = "linux")).
vm/devices/net/net_tap/Cargo.toml	Switch target-specific deps/dev-deps from cfg(unix) to Linux-only.
vm/devices/net/net_backend_resources/src/lib.rs	Change TapHandle to carry OwnedFd; gate the tap module to Linux.
openvmm/openvmm_entry/src/ttrpc/mod.rs	Open TAP in the entrypoint for ttrpc/grpc NIC configs and pass fd via TapHandle.
openvmm/openvmm_entry/src/lib.rs	Open TAP in CLI parsing on Linux; produce a clear error on non-Linux.
openvmm/openvmm_entry/Cargo.toml	Add net_tap dependency only on Linux.
Cargo.lock	Lockfile updated to include net_tap as a dependency of openvmm_entry.

[chris-oo]

Looks reasonable to me, but do we need the tap filename as part of the context, or is it part of the upper chained context?

[jstarks]

Added the context to the errors.