rust: introduce alternative implementation of offset_of!

offset_of! was stabilized in Rust 1.77.0. Use an alternative implemenation
that was found on the Rust forums, and whose author agreed to lic

rust: introduce alternative implementation of offset_of!

offset_of! was stabilized in Rust 1.77.0. Use an alternative implemenation
that was found on the Rust forums, and whose author agreed to license as
MIT for use in QEMU.

The alternative allows only one level of field access, but apart
from this can be used just by replacing core::mem::offset_of! with
qemu_api::offset_of!.

The actual implementation of offset_of! is done in a declarative macro,
but for simplicity and to avoid introducing an extra level of indentation,
the trigger is a procedural macro #[derive(offsets)].

The procedural macro is perhaps a bit overengineered, but it helps
introducing some idioms that will be useful in the future as well.

Signed-off-by: Junjie Mao <junjie.mao@hotmail.com>
Co-developed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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rust: create a cargo workspace

Workspaces allows tracking dependencies for multiple crates at once,
by having a single Cargo.lock file at the top of the rust/ tree.
Because QEMU's Cargo.lock files h

rust: create a cargo workspace

Workspaces allows tracking dependencies for multiple crates at once,
by having a single Cargo.lock file at the top of the rust/ tree.
Because QEMU's Cargo.lock files have to be synchronized with the versions
of crates in subprojects/, using a workspace avoids the need to copy
over the Cargo.lock file when adding a new device (and thus a new crate)
under rust/hw/.

In addition, workspaces let cargo download and build dependencies just
once. While right now we have one leaf crate (hw/char/pl011), this
will not be the case once more devices are added.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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