rust: allow using build-root bindings.rs from cargo

rust: allow using build-root bindings.rs from cargo

Right now, using cargo with QEMU requires copying by hand the bindings.rs to the
source tree.  Instead, we can use an include file to escape the cage of cargo's
mandated source directory structure.

By running cargo within meson's "devenv" and adding a MESON_BUILD_ROOT
environment variable, it is easy for build.rs to find the file.  However, the
file must be symlinked into cargo's output directory for rust-analyzer to find
it.

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