refactor: use matches! instead of to_string() for tests

Some tests that were explicitly testing for error conditions used
converted errors to strings to determine whether two errors are the same
(by saying they're only the same if their string representation was
identical). Replace this with more roboust assertions on `matches!`.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

CHANGELOG.md

@@ -14,6 +14,8 @@
   `write_volatile_to` and `write_all_volatile_to` functions from the `GuestMemory` trait to the `Bytes` trait.
 - \[[#312](https://github.com/rust-vmm/vm-memory/pull/312)\]: Give `GuestMemory::find_region` and `GuestMemory::num_regions`
   a default implementation, based on linear search.
+- \[[#312](https://github.com/rust-vmm/vm-memory/pull/312)\]: Provide a marker trait, `GuestMemoryRegionBytes`, which enables a default implementation of `Bytes<MemoryRegionAddress>`
+  for a `GuestMemoryRegion` if implemented.
 
 - \[#324](https:////github.com/rust-vmm/vm-memory/pull/324)\] `GuestMemoryRegion::bitmap()` now returns a `BitmapSlice`. Accessing the full bitmap is now possible only if the type of the memory region is know, for example with `MmapRegion::bitmap()`.
 

src/lib.rs

@@ -51,7 +51,9 @@ pub use guest_memory::{
 };
 
 pub mod region;
-pub use region::{GuestMemoryRegion, GuestRegionCollection, GuestRegionError as Error};
+pub use region::{
+    GuestMemoryRegion, GuestMemoryRegionBytes, GuestRegionCollection, GuestRegionError as Error,
+};
 
 pub mod io;
 pub use io::{ReadVolatile, WriteVolatile};