bitmap: Add bitmap_copy_with_{src|dst}_offset()

These helpers copy the source bitmap to destination bitmap with a
shift either on the src or dst bitmap.

Meanwhile, we never have bitmap tests but we should.

This patch also introduces the initial test cases for utils/bitmap.c
but it only tests the newly introduced functions.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20190603065056.25211-5-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

---

Bitmap test used sizeof(unsigned long) instead of BITS_PER_LONG.