block: vhdx: Remove catch all in physical_size boundary

Vhdx::physical_size() can only return Error::GetFileMetadata.
Replace the catch-all arm with unreachable!() so future error
variants are not silently mapped to a generic Io classification.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/vhdx_sync.rs

@@ -53,7 +53,7 @@ impl disk_file::PhysicalSize for VhdxDiskSync {
                 Error::GetFileMetadata(io) => {
                     BlockError::new(BlockErrorKind::Io, Error::GetFileMetadata(io))
                 }
-                _ => BlockError::new(BlockErrorKind::Io, e),
+                _ => unreachable!("unexpected error from Vhdx::physical_size(): {e}"),
             })
     }
 }