hbitmap: Add hbitmap_is_serializable()

Bitmaps with a granularity of 58 or above can be neither serialized nor
deserialized (see the comment in the function added in this series for
an explanation). This patch adds a function so that we can check whether
a bitmap actually can be (de-)serialized at all, thus avoiding failing
the necessary assertion in hbitmap_serialization_granularity().

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20161115225746.3590-2-mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>