migration/dirty-bitmaps: change bitmap enumeration method

Shift from looking at every root BDS to *every* BDS. This will migrate
bitmaps that are attached to blockdev created nodes instead of just ones
attached to emulated storage devices.

Note that this will not migrate anonymous or internal-use bitmaps, as
those are defined as having no name.

This will also fix the Coverity issues Peter Maydell has been asking
about for the past several releases, as well as fixing a real bug.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Reported-by: Coverity 😅


Reported-by: aihua liang <aliang@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 20190514201926.10407-1-jsnow@redhat.com
Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1652490


Fixes: Coverity CID 1390625
CC: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>