vfio_user: Workaround 'destructuring_assignment' for rust 1.56

The 'destructuring_assignment' to tuples [1] was not stable with rust
toolchina 1.56.

[1] rust-lang/rust#71126

Signed-off-by: Bo Chen <chen.bo@intel.com>

vfio_user/src/lib.rs

@@ -479,10 +479,14 @@ impl Client {
         for index in 0..num_regions {
             let (mut region_info, mut fd, mut sparse_areas) = self.get_region_info(index, None)?;
             if region_info.argsz > std::mem::size_of::<vfio_region_info>() as u32 {
-                (region_info, fd, sparse_areas) = self.get_region_info(
+                // Note: workaround for https://github.com/rust-lang/rust/issues/71126 to compile with rust toolchian 1.56
+                let (_region_info, _fd, _sparse_areas) = self.get_region_info(
                     index,
                     Some(region_info.argsz - std::mem::size_of::<vfio_region_info>() as u32),
                 )?;
+                region_info = _region_info;
+                fd = _fd;
+                sparse_areas = _sparse_areas;
             }
             regions.push(Region {
                 flags: region_info.flags,