rust: add module to convert between success/-errno and io::Result

It is a common convention in QEMU to return a positive value in case of
success, and a negated errno value in case of error.  Unfortunately,
using errno portably in Rust is a bit complicated; on Unix the errno
values are supported natively by io::Error, but on Windows they are not;
so, use the libc crate.

This is a set of utility functions that are used by both chardev and
block layer bindings.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/src/assertions.rs

@@ -92,3 +92,31 @@ macro_rules! assert_field_type {
         };
     };
 }
+
+/// Assert that an expression matches a pattern.  This can also be
+/// useful to compare enums that do not implement `Eq`.
+///
+/// # Examples
+///
+/// ```
+/// # use qemu_api::assert_match;
+/// // JoinHandle does not implement `Eq`, therefore the result
+/// // does not either.
+/// let result: Result<std::thread::JoinHandle<()>, u32> = Err(42);
+/// assert_match!(result, Err(42));
+/// ```
+#[macro_export]
+macro_rules! assert_match {
+    ($a:expr, $b:pat) => {
+        assert!(
+            match $a {
+                $b => true,
+                _ => false,
+            },
+            "{} = {:?} does not match {}",
+            stringify!($a),
+            $a,
+            stringify!($b)
+        );
+    };
+}