test-hbitmap: Add hbitmap_is_serializable() calls

Add calls to hbitmap_is_serializable() (asserting that it returns true)
where necessary (i.e. before every series of (de-)serialization function
invocations).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20161115225746.3590-3-mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>