chore: Enable clippy::assertions_on_result_states

As @roypat pointed out, and quote:

"This lint disallows asserttions on is_ok()/is_err() in favor of either
using unwrap (so that at least if the test fails, we the failure message
will contain the actual failure reason instead of just "was not
ok/err"), or actually matching the specific variant."

Signed-off-by: Ruoqing He <[email protected]>

Author: RuoqingHe

src/lib.rs

@@ -7,6 +7,7 @@
 #![deny(missing_docs)]
 #![deny(missing_copy_implementations)]
 #![deny(missing_debug_implementations)]
+#![warn(clippy::assertions_on_result_states)]
 
 //! A safe wrapper around the kernel's KVM interface.
 //!