rust: qemu-api: add bindings to Error

Provide an implementation of std::error::Error that bridges the Rust
anyhow::Error and std::panic::Location types with QEMU's Error*.

It also has several utility methods, analogous to error_propagate(),
that convert a Result into a return value + Error** pair.  One important
difference is that these propagation methods *panic* if *errp is NULL,
unlike error_propagate() which eats subsequent errors[1].  The reason
for this is that in C you have an error_set*() call at the site where
the error is created, and calls to error_propagate() are relatively rare.

In Rust instead, even though these functions do "propagate" a
qemu_api::Error into a C Error**, there is no error_setg() anywhere that
could check for non-NULL errp and call abort().  error_propagate()'s
behavior of ignoring subsequent errors is generally considered weird,
and there would be a bigger risk of triggering it from Rust code.

[1] This is actually a violation of the preconditions of error_propagate(),
    so it should not happen.  But you never know...

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

docs/devel/rust.rst

@@ -96,6 +96,11 @@ are missing:
   architecture (VMState).  Right now, VMState lacks type safety because
   it is hard to place the ``VMStateField`` definitions in traits.
 
+* NUL-terminated file names with ``#[track_caller]`` are scheduled for
+  inclusion as ``#![feature(location_file_nul)]``, but it will be a while
+  before QEMU can use them.  For now, there is special code in
+  ``util/error.c`` to support non-NUL-terminated file names.
+
 * associated const equality would be nice to have for some users of
   ``callbacks::FnCall``, but is still experimental.  ``ASSERT_IS_SOME``
   replaces it.