block: vhd: fix incomplete bounds check in sync I/O worker

The sync I/O worker only checked that the operation offset did
not start past the end of the virtual disk (offset >= size) -
did not verify that the operation end (offset + len) stays
within bounds.

A read or write that started inside the image but extended
beyond the logical size was silently passed to the raw backend.

The async io_uring worker already had the correct check
(offset + len > size with overflow protection). I extracted it
into a shared helper in worker/common.rs and reused inside the
sync path to eliminate duplication and close the gap.

Fixes #8311

Signed-off-by: Alexander Lvov <alexander.lvov.git@gmail.com>

block/src/formats/vhd/worker/mod.rs

@@ -9,4 +9,5 @@
 
 #[cfg(feature = "io_uring")]
 pub(crate) mod async_uring;
+mod common;
 pub(crate) mod sync;