rust: repurpose qemu_api -> tests

rust: repurpose qemu_api -> tests

The crate purpose is only to provide integration tests at this point,
that can't easily be moved to a specific crate.

It's also often a good practice to have a single integration test crate
(see for ex https://github.com/rust-lang/cargo/issues/4867)

Drop README.md, use docs/devel/rust.rst instead.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Link: https://lore.kernel.org/r/20250827104147.717203-20-marcandre.lureau@redhat.com
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>