add naive default impl for GuestMemory::num_regions

Similar to find_region(), we can provide an inefficient default impl to
make downstream impls of the trait less verbose in scenarios that really
do not care (e.g. mock impls for tests).

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

Author: roypat

CHANGELOG.md

@@ -10,8 +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.
+- \[[#312](https://github.com/rust-vmm/vm-memory/pull/312)\]: Give `GuestMemory::find_region` and `GuestMemory::num_regions`
+  a default implementation, based on linear search.
 
 - \[#324](https:////github.com/rust-vmm/vm-memory/pull/324)\] `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

@@ -405,7 +405,9 @@ pub trait GuestMemory {
     type R: GuestMemoryRegion;
 
     /// Returns the number of regions in the collection.
-    fn num_regions(&self) -> usize;
+    fn num_regions(&self) -> usize {
+        self.iter().count()
+    }
 
     /// Returns the region containing the specified address or `None`.
     fn find_region(&self, addr: GuestAddress) -> Option<&Self::R> {