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>

docs/devel/rust.rst

@@ -179,6 +179,7 @@ module           status
 ``callbacks``    complete
 ``cell``         stable
 ``c_str``        complete
+``errno``        complete
 ``irq``          complete
 ``memory``       stable
 ``module``       complete