Add naive default impl for GuestMemory::find_region()

This function can be default-implemented in terms of
`GuestMemory::iter()`. Downstream impls can overwrite this more
specialized and efficient versions of course (such as GuestMemoryMmap
using a binary search).

Mainly a QoL improvement to make mock impls of this trait for tests
less verbose.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Author: roypat

CHANGELOG.md

@@ -10,6 +10,8 @@
 
 - \[[#307](https://github.com/rust-vmm/vm-memory/pull/304)\] Move `read_volatile_from`, `read_exact_volatile_from`,
   `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` a default implementation,
+  based on linear search.
 
 - \[#324](https:////github.com/rust-vmm/vm-memory/pull/324)\] `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/guest_memory.rs

@@ -408,7 +408,9 @@ pub trait GuestMemory {
     fn num_regions(&self) -> usize;
 
     /// Returns the region containing the specified address or `None`.
-    fn find_region(&self, addr: GuestAddress) -> Option<&Self::R>;
+    fn find_region(&self, addr: GuestAddress) -> Option<&Self::R> {
+        self.iter().find(|region| addr >= region.start_addr() && addr <= region.last_addr())
+    }
 
     /// Gets an iterator over the entries in the collection.
     ///