rust: make declaration of dependent crates more consistent

rust: make declaration of dependent crates more consistent

Crates like "bilge" and "libc" can be shared by more than one directory,
so declare them directly in rust/meson.build.  While at it, make their
variable names end with "_rs" and always add a subproject() statement
(as that pinpoints the error better if the subproject is missing and
cannot be downloaded).

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